Programming and Application of a DSP to Control and Regulate Power Electronic Converters: Programming in C++
©2014
Textbook
146 Pages
Summary
The purpose of this project has been to study, operate and program the 32-bit 150MIPS TMS320F2812 DSP developed by Texas Instruments Inc. In addition, it has also been a goal to implement fast estimation techniques for control of resonant converters. For this purpose, PWM signals that are generated using this DSP are used. The demands on the system and the hardware to solve the problem were already decided when I started the work.<br>The algorithms were programmed in C/C++ language, compiled, debugged and transferred to the DSP development board in a compiling and simulation tool (downloader), called CCS (Code Composer Studio v2), also provided by Texas Instruments.<br>In the first chapters of this study I give general information about control systems, digital signal processors, digital signal processing and the DSP used in this work. The following chapters tell about PWM, how to configure the PWM outputs and some examples related with PWM signals are given. After a short review of series resonant converters, I presented the last example implemented in this project. I conclude with a summary and provide some hints of future work.
Excerpt
Table Of Contents
List of Figures & Tables
Figures
Figure 1.1 Power Electronic System Consisting of Power Electronics and Control
7
Figure 1.2 Electrical Drive Consisting of Power Electronics, Electrical Machine and Control
9
Figure 1.3 Basic Structure of a Typical Motion Control System
11
Figure 1.4 Typical Microcontroller-Based Digital Control System Diagram for PWM DC/DC Converter 18
Figure 1.5
Typical DSP-Based Digital Control System Diagram for PWM DC/DC Converter
19
Figure 1.6
Architecture of Digital PWM Using Digital Signal Processor
20
Figure 1.7 MAC Operation
22
Figure 1.8 In DSPs, an Analog Signal such as Voice is Digitized by an Analog-to-Digital Converter
23
Figure 1.9
Harvard Architecture
23
Figure 1.10 Von Neumann Architecture
24
Figure 2.1 A New TI DSP Product Line: 32-bit Flash Mixed Signal DSP
34
Figure 2.2 C28x DSP Core
35
Figure 2.3 On-Chip 12-bit Analog-to-Digital Converter
36
Figure 2.4 2 On-Chip Event Managers
37
Figure 2.5 TMS320F2812 DSP Simplified Hardware Diagram
37
Figure 2.6
IQmath Library: Floating Point on a Fixed Point Machine
38
Figure 2.7 IQmath Approach
38
Figure 2.8 Block Diagram of the F2812 ADC Module
42
Figure 3.1 Block Diagram of the eZdsp F2812
46
Figure 3.2 eZdsp F2812 Connector Positions
47
Figure 3.3 Top View of the eZdsp F2812 Board
47
Figure 3.4
P1 Pin Locations
48
Figure 3.5 Connector P2 Pin Locations
48
Figure 3.6 P4/P8/P7 Connectors
49
Figure 3.7 Connector P5/P9 Pin Locations
51
Figure 4.1
Steps Taken during DSP Software Development
53
Figure 4.2 The Project Creation Window
54
Figure 4.3 The Project Manager Window
56
Figure 4.4 The Watch Window
59
Figure 4.5 The File I/O Dialog
61
Figure 4.6 Changing the Address and Length Values in the File I/O Dialog
61
Figure 4.7 The Brake/Probe Points Dialog
62
Figure 4.8 The Graph Property Dialog
63
Figure 5.1 Decomposition of PWM Signal
65
Figure 5.2 Asymmetric PWM Waveform Generation with Compare Unit and PWM Circuits
69
Figure 5.3 Symmetric PWM Waveform Generation with Compare Units and PWM Circuits
69
Figure 5.4 PWM Signal and its Complement at the Pins PWM3&4 and PWM9&10
74
Figure 5.5 PWM Signal and its Complement at the Pins PWM5&6 and PWM11&12
75
Figure 5.6 PWM Signals plus Complemented PWM with Dead-Time
77
Figure 5.7 PWM Signals plus PWM with Dead-Time
77
Figure 5.8 PWM1 & PWM2 without Dead Band: PWM1 Active High and PWM2 Active Low
78
Figure 5.9 PWM1 (Active High) and PWM2 (Active Low) Outputs with Dead Band
78
Figure 5.10 PWM1 (Active Low) and PWM2 (Active High) Outputs with Dead Band
79
Figure 5.11 Both PWM1 and PWM2 Configured Active High with Dead Band
79
Figure 5.12 Both PWM1 and PWM2 Configured As Active Low with Dead Band
79
Figure 5.13 Inverter Phase with Two Power Devices Connected in Series
80
Figure 5.14 PWM Signal with Frequency=30kHz and Duty Cycle=50% and its Complement
82
Figure 5.15 The Experimental Board
83
Figure 5.16 Register Watch Window in CCS
84
Figure 5.17 PWM with f=20kHz, Duty Cycle=50%, without Dead-Band
85
Figure 5.18 PWM with f=30kHz, Duty Cycle=50%, without Dead-Band
85
Figure 5.19 PWM with f=40kHz, Duty Cycle=50%, without Dead-Band
86
Figure 5.20 PWM with f=20kHz, Duty Cycle=90%, without Dead-Band
86
Figure 5.21 PWM with f=20kHz, Duty Cycle=70%, without Dead-Band
86
Figure 5.22 PWM with f=20kHz, Duty Cycle=50%, Dead-Band=6.4s
88
Figure 5.23 PWM with f=20kHz, Duty Cycle=50%, Dead-Band=4.4s
88
Figure 5.24 PWM with f=20kHz, Duty Cycle=50%, Dead-Band=2.4s
88
Figure 5.25 PWM with f=1kHz, 50% Duty Cycle and Dead-Band=448s
89
Figure 6.1 Duty Cycle Modulation versus Analog Input Voltage
90
Figure 6.2 Sine-Modulated PWM Signal
91
Figure 6.3 Adding a File to the Probe
92
Figure 6.4 Connecting the Probe Point to a File
92
Figure 6.5 Plot of the Stored Sine Values in File
93
Figure 6.6 The Potentiometers and DIP Switches on the Board
94
Figure 6.7
Sine Modulated PWM to Control Inverters
95
Figure 6.8 PWM with F=9.5kHz, DIP#1DIP#2=01
97
Figure 6.9 PWM with F=7.4kHz, DIP#1DIP#2=10
97
Figure 6.10 PWM with F=120kHz, DIP#1DIP#2=11
97
Figure 6.11 The Converter Circuit
98
Figure 6.12 Electrical Circuit of the Series Resonant DC-DC Converter
99
Figure 6.13 Series Resonant DC-DC Converter Equivalent Circuit
100
Figure 6.14 Transformer Set Up with a Fixed Primary Coil and a Movable Secondary Coil
101
Figure 6.15 Sampling of the Sinusoidal Current
102
Figure 6.16 Converter Frequency is Higher than the Switching Frequency
105
Figure 6.17 Converter Frequency is Equal to the Switching Frequency
105
Figure 6.18 Converter Frequency is Lower than the Switching Frequency
106
Figure 6.19 SRC Circuit with Snubber Capacitor C
S
Added Parallel to T1
108
Figure 6.20 Commutating the Transistor Voltage Linearly from 0 to U
in
108
Figure 6.21 UT1(1) and Converter Current(2) when Dead-Time=0.5s, without Capacitor
109
Figure 6.22 UT1(1) and Converter Current(2) when Dead-Time=3s
109
Figure 6.23 UT1(1) and Converter Current(2) when Dead-Time=0.5s
110
Figure B.1 Circuit Used in Control of SRC
134
Figure B.2 Photograph of the SRC Set-Up
134
Figure B.3 Circuit of the Experimental Set-Up
135
Figure B.4 Photograph of the Experimental Set-Up
135
Figure D.1 Schematic of the DSP
138
Figure D.2 Schematic of the P1 Header
138
Figure D.3 Schematic of the P2 Header
139
Figure D.4 Schematic of the P3 Parallel Interface
139
Figure D.5 Schematic of the P4 Header
140
Figure D.6 Schematic of the P5 Header
140
Figure D.7 Schematic of the P6 Power Connector
140
Figure D.8 Schematic of the P7 Header
141
Figure D.9 Schematic of the P8 Header
141
Figure D.10 Schematic of the P9 Header
141
Figure D.11 Schematic of the JTAG Controller
142
Tables
Table 1.1
Evolution of DSPs
32
Table 3.1
eZdsp F2812 Connectors
48
Table 3.2
P8, I/O Connectors
50
Table 3.3
P9, Analog Interface Connector
51
Table 5.1
Deadband Register Settings for Dead-Band Generation
87
Table 6.1
States of DIP Switches in Prg a
94
Table 6.2
States of DIP Switches in Prg b
96
Table 6.3
SRC Operating Modes
100
Table E.1
The Sine Values Stored in File
143
7
1. Introduction
1.1 Power Electronic and Electrical Drive Systems
The market for power electronics and power-electronic-controlled electrical drives is rapidly
growing. Often, the application of power electronic and electrical drive (PE&ED) systems
requires careful engineering because PE&ED systems are utilized as energy converters or as
actuators embedded in larger engineering systems. Control systems are required to obtain the
desired characteristics of the PE&ED systems and the entire application. The diversity of
applications is reflected in a correspondingly wide range of control methods. Hence, a large
variety and a large volume of control systems have to be designed, implemented and tested.
1.1.1 Power Electronic Applications
Power electronics constitutes an electrical engineering system, which is always embedded in a
system comprising an electric power supply and an electrical load as depicted in Figure 1.1.
Note that power electronics itself does not constitute a source of electric power but transforms
electrical energy. Electrical energy can be supplied by the electric utility grid comprising
remote energy sources and transmission devices. Alternatively, it might be supplied by local
energy sources, such as solar cells, wind or hydro generators or energy storage elements like
electrochemical batteries [1].
Figure 1.1 Power Electronic System Consisting of Power Electronics and Control Embedded between a Power
Supply and an Electrical Load
If the characteristics of the energy source do not meet the requirements of the load then power
electronics is used as an interface between the energy source and the load. This might for
instance be related to the amplitude of dc systems, the amplitude and frequency of fixed
frequency ac systems or disturbances caused by other loads connected to the same power
supply. Thus, the task of power electronics can be defined as converting electric energy and
controlling the flow of electric power by means of power semiconductor devices in
accordance with the requirements of the load. The latter may comprise specifications on
Power
Supply
Electrical
Load
Power
Electronics
8
amplitude, frequency, and number of phases, allowed harmonic distortion and transient
voltages. High efficiency is most often a major requirement for power electronic circuits due
to the cost of energy and cooling systems.
Power electronics covers a wide range of applications including the following [1]:
·
Energy conversion as part of electrical drives
·
DC/DC converters of virtual any power range
·
Unity-power-factor rectifiers for applications including railway traction drives
which operate of a single-phase ac catenary supply
·
Generation of (multi-phase) voltage systems of virtually any frequency or
amplitude
·
Power engineering applications such as `flexible ac transmission systems'
optimizing the utilization of electrical power transmission equipment
Most current power electronics (PE) systems are implemented using an intermediate dc link
between two stages of power conversion. Depending on the energy storage element used,
these systems impress voltages or currents on their loads and are thus called voltage source
inverters (VSIs) or current source inverters (CSIs), respectively.
1.1.2 Switched Mode Operation
`Switched mode operation' is applied by power electronic circuits due to efficiency
requirements and feasibility. In contrast, linear electronic systems use semiconductor devices
as adjustable resistors by operating them in their linear region. This results in a low energy
efficiency, which is not tolerable or even feasible at high energy levels or the energy density
prevailing in power electronics [2]. For this reason, power electronics embodies power
semiconductors used as switches that are either (ideally) fully on or fully off. The power
semiconductors are operated that way that they switch between conducting and blocking
states in a cyclic manner. The high efficiency achieved by this `switched mode' operation is
quite important due to the cost of wasted energy and the difficulty of removing the generated
heat.
Most applications of switched mode operation involve the control of mean values to achieve
the desired characteristics. Hence, the ratio of the on-state time of a semiconductor to the total
9
cycle time is controlled. It is an important issue in control of power electronics to maximize
the control bandwidth despite a limited switching frequency. The switching frequency usually
has to be limited because the allowable total losses of the semiconductor devices have to be
restricted. Besides off-state leakage and on-state conduction losses, the losses are related to
switching losses caused by non-zero voltage and current during the finite duration of the
switching operation. The total losses are most often limited by the heat dissipation capability
of the package and the permissible power semiconductor junction temperature. A finite
switching frequency causes VSI load currents or CSI load voltages to oscillate around the
desired mean values.
1.1.3 Electrical Drive Applications
An electrical drive (ED) can be defined as a controlled electromechanical actuator for the
purpose of `motion control' of a larger mechanical engineering plant as shown in Figure 1.2.
It consists of the electrical machine, the power converter, the control equipment and the
mechanical load [3]. In all drives where the speed and position are controlled, a power
electronic converter is needed as an interface between the input power and the motor [4].
Figure 1.2 Electrical Drive Consisting of Power Electronics, Electrical Machine and Control Embedded between
a Power Supply and a Mechanical Load
It has to be emphasized that power electronic is used as tool to provide electric energy to
electromechanical actuators for their movement and to achieve control of behavior of the
actuators, thus achieving control of the behavior of the mechanical load. Thereby, the
characteristics of the power supply to the electrical machine are used to control motion.
Examples of variable speed drive or motion control applications are:
·
variable-speed drives for household appliances or industrial purposes
·
high-performance servo drives for factory automation
·
railroad traction drives
·
maritime electrical propulsion systems
Power
Supply
Electrical
Machine
Power
Electronics
Mechanical
Load
10
1.1.3.1 Motion Control
Motion control deals with the use of high performance electric motors and is a very important
part of industrial control systems. Motion control includes applications for speed and torque
or position control in practically all branches of industry. An important advance in this field
has been made during the last years by the introduction of microprocessor control systems.
These systems are becoming a standard in motion control because of fast advances in
microelectronics technology and well-known benefits, such as: [5,6,7]
·
Greater accuracy
·
Flexibility
·
Repeatability
·
Less noise
·
Parameter sensitivity
·
Higher interconnection capacity
As a consequence of this progress, more and more applications that have used simple electric
drives for economic reasons are being replaced by motion control systems. This is a natural
evolution considering the benefits offered by motion control systems in meeting the ever-
increasing needs for improved quality and greater productivity in all industries.
To facilitate this move towards motion control solutions, efforts are being made to
continuously decrease the cost of these systems, especially considering their power electronics
and control parts.
Motion Control System Requirements
Figure 1.3 presents the basic structure of a typical motion control system, which consists of:
·
Electric motor (M)
·
Power electronic block (PE)
·
Digital control system (DCS)
11
Figure 1.3 Basic Structure of a Typical Motion Control System
The inputs to the digital controller are the reference quantity y and the process values, which
are measured with a set of sensors and generally converted using analogue-to-digital (A/D)
converters. These values are electrical (voltage V, current I) and mechanical (velocity w,
position q) signals.
The DCS controls the PE through an output interface (OI). This interface consists of digital to
analogue (D/A) converters, if the PE requires an analogue input, or of a simple isolation
module, if the DCS directly controls the PE switching elements (transistors, IGBTs, etc.).
The requirements for such a system are demanding regarding steady state accuracy and
dynamic behavior, sensitivity to external disturbances, measurement noise and parameter
variation, and especially reliability and cost.
Inevitably, more and more quantities must be controlled simultaneously (position and/or
speed, acceleration, torque, and current). All of these quantities are changing rapidly enough
that the sampling time must be very small (typically in the range of 0.1 to 1 ms). Some of
these quantities are measured; others can be estimated.
Invariably, there are analog inputs. The number of inputs can range from 5 to 8 for complex
systems. This means that the microprocessor system must read the information from the
corresponding sensors. Precision depends on the accuracy of the sensors and the converters.
Typically these converters must have 10 or more bits of accuracy and a conversion time
maximum of 10 ms.
DCS
OI
PE
sensors
M
V, I
,
y
y*
-
A/D
Note:
DCS digital control system; M electric motor; PE power electronics; OI output interface
12
The extreme demands imply that sophisticated control algorithms must be executed in each
sampling step. The control scheme of motion control systems is typically complex, even if the
controller block is a simple PID (as it is in numerous applications).
For example, in the popular induction motor control principle the complexity derives from the
induction motor, which is a nonlinear element. To linearize the induction motor, the equations
must be reported to the rotor flux frame. The reference currents obtained in this way must be
expressed in the stator frame so that the PE can be controlled. Control diagram complexity
differs with the motor type and the system performance.
In many cases the simple PID controller is replaced by more sophisticated schemes, such as
adaptive or optimal controllers, to obtain an improved behavior even when parameters change
or disturbances occur.
Furthermore, the control scheme can become more complex if quantities necessary for control
are estimated rather than measured. In this way the corresponding sensors are eliminated and
the system becomes cheaper and more robust.
If analog control is completely eliminated, the digital control system must generate the
switching commands for the PE elements. Depending on the motor and PE type, these
commands must be computed following a specific algorithm.
For example, the pulse width modulation (PWM) strategy is usually employed for AC motors.
The output value can be converted to the corresponding switching sequence either by software
or hardware. The hardware solution is more complex but faster.
Aside from the motion control, the microprocessor system must communicate with other
systems to receive commands and return process information. This usually requires a special
motion control language consisting of a collection of instructions processed by the control
system. The microprocessor thus receives the reference not as an array of values but as
concise information used to construct an exact profile.
13
A DSP is frequently used to accomplish such a large number of computations in the small
sampling time (0.1 to 1 ms) provided. The DSP solution offers sophisticated control schemes
with very good performance.
Because cost is an important restriction for most industrial applications, the scheme must be
carefully chosen. An optimum solution must be balanced between the requirement for high
performance and the cost of the microprocessor control system.
The cost of a DSP control system depends not only on the cost of the DSP but also the cost of
external components (converters, filters, memory, specialized inputs and outputs such as an
encoder and a PWM) [8].
A strong tendency in the field of motion control is to integrate the motor, the power converter,
and the control electronics in a single, compact unit. This is possible because the size of
power electronics is constantly being reduced. An intelligent motor is thus obtained with an
extremely simple implementation, increased reliability, and reduced cost [9].
14
1.2 Control Systems
Control intends to impose certain behavior on an engineering system (plant) in order to satisfy
control criteria such as disturbance rejection, low steady-state error, fast transient response
and robustness to parameter changes in the plant etc [10]. Control is used to ensure and
increase the quality of the performance of the plant. The implementation of a control system
or the emulation of a control design by rapid prototyping methods requires appropriate
hardware structures.
1.2.1 Digital versus Analog Implementation
In power electronics systems, output voltage regulation has traditionally been accomplished
using analog control concepts. In the analog controller, the analog signals of the output
voltage and/or current are first processed by an analog transfer function using an operational
amplifier with its appropriate compensation network. In this way, the transfer function applies
the control laws to shape the output response of the switching converter. The analog controller
then adjusts the duty cycle or switching frequency to drive the power switching devices. The
main advantage of the analog control system is that the system operates in real time and can
have very high bandwidth. Also, the voltage resolution of an analog system is theoretically
infinite. However, an analog system is usually composed of hardware that does not lend itself
to design changes, and advanced control techniques used to improve performance require an
excessive number of analog components.
Even though dedicated analog integrated circuits remain the workhorse of controllers for
switching converters, digital controllers are finding more and more applications. It is not only
the steady price reduction that has made them attractive in various applications, but also the
great functional developments of microcontrollers and digital signal processors (DSPs). As
shown in Figures 1.4 and 1.5, the digital controller accepts the digitized sampled output
voltage and current through analog-to-digital converters (ADCs). This information is then
processed using a digital control algorithm, which is similar to the analog compensation
network in terms of function. The output of the digital controller is then used to drive the
power switching devices. In general, digital control offers some advantages over the analog
counterpart. Some of the advantages are as follows:
15
·
Digital components are less susceptible to aging and environmental variations.
·
They are less sensitive to noise.
·
Changing a controller does not require an alteration in the hardware.
·
They can provide monitoring, self-diagnostics, and communication with a host computer
or among several digital controllers.
·
The most important is they can facilitate some advanced control techniques, such as
space vector modulation, adaptive control, fuzzy control, etc.
However, digital control systems are not without disadvantages when compared with analog
control systems. Some of the disadvantages are as follows:
·
Finite signal resolution due to the finite wordlength of the ADCs, and DACs or PWM
outputs, which cause the output less accurate. Analog control
gives infinite resolution of
the measured signal.
·
Time delays in the control loop due to both the sampling of ADCs and computation of the
control algorithm by the processors.
·
A
nalog control can provide continuous processing of signal, thus allowing very high
bandwidth.
·
It may still need some analog interface circuits, increasing the overall system complexity
and cost. However, this problem can be alleviated with more functions integrated into the
controller or even application specific integrated circuits (ASICs).
Actually, DSPs or Microcontrollers are already widely used in AC motor drives and
uninterruptible power supplies (UPSs), which are both inverter-based systems usually
working at a few tens of kHz. They have permitted the application of advanced control
techniques in these systems, which is hard to implement with analog control otherwise.
A digital AC drive consists of implementation of torque control by means of regulating the
motor current which requires very high speed computation in the range of tens of
microseconds, and speed control which requires relatively moderate computation intensity in
the range of hundreds microseconds. All these functions have been implemented in one DSP
or one microcontroller with/without a separate motion ASIC. This popularity has been due
partly to availability and flexibility of desired algorithm implementation.
16
1.2.1.1 Review of Today's Servo Drive Systems
Today's most servo motor drive systems are implemented by digital closed loop control
instead of analog control. This has been primarily due to rapid advancement of Digital Signal
Processor (DSP) and microcontrollers applied to motor control applications. In a typical servo
control system, several functions are divided into tasks which run at different update rates
depending on the required bandwidth and nature of the processing priority need real-time
operation versus delayed batch processes, scanned tasks versus one-time event driven tasks.
Each task is controlled by a multitask operating system closely coupled with DSP or
microcontroller interrupt structure.
A servo drive system in terms of a functional element, which deals with much closer machine
control, requires fast processing, fast update rate and real-time process. They are closely tied
with a specific motion peripheral hardware and it sometimes requires specific coding unique
to peripheral hardware and interrupt structure inside of DSP or microcontroller.
On the contrary, tasks that are far apart from the machine side and are close to the host
communication or man-machine interface side, require less frequent update and slow
processing. However, it requires more memory intensive calculation since reference command
generation over controlling parameter is more complicated than those, which are close to the
machine side. For example, position reference command is much more complex as
sophisticated motion profile generation advances. However, torque command is produced in a
simple step function.
The fact is that torque is a fastest machine parameter, and needs to be controlled much more
quickly than speed of motor shaft. Integral of torque is speed. Integral of speed is position.
Integral of power results in motor temperature rise. Because of this chain of physical motor
parameters, each parameter requires different speed of processing. It is typical that a real-time
multi-tasking operating system is used to satisfy each required processing power [11].
Although digital control has been widely applied to motor drives and UPS applications, the
digital control of power supplies faces slightly different technical challenges. In the case of
motor drives, the controlled variable of interest is a mechanical quantity, such as position or
velocity. Electrical dynamics are often included in the overall design model to achieve high
17
performance goals, but the dominant time constants are associated with mechanical dynamics
and are relatively large. Sampling periods are often on the order of several milliseconds to
tens of milliseconds, so it is relatively easy to complete control calculations within a sampling
period. In addition, the switching frequency is also about one order of magnitude lower than
that of most power supplies. Therefore, it is also relatively easy to implement real time PWM
control with the-state-of-art digital processors.
In the contrast, power supply applications focus on control of an electrical quantity, such as
output voltage. Objectives often include excellent rejection of input and load of variations.
The time constants of interest are often several orders of magnitude smaller than for motor
drives. Hence, the higher sampling frequency is of greater concern.
Therefore, analog control concept is still the workhorse of most DC/DC converters. For most
applications, especially low power DC/DC power supplies, analog control, which is usually
realized by a single PWM control chip, still have advantages in terms of cost and simplicity.
However, as the applications of power electronics are getting broader and the power
electronics systems themselves more complex, the complexity of the system is beyond the
capacity of analog control or the performance hardly satisfactory in some applications, e.g.,
some battery chargers, automotive HID ballast, voltage regulator modules (VRMs).
A microcontroller can be very inexpensive and possess many I/O functions (such as ADCs,
timers, etc.), but its computing performance is often significantly lower than a typical DSP.
For example, microcontroller may not support a fast multiplication or division instruction not
to mention the shorter wordlength of the processor and longer machine cycle even for the
same clock rate. Though DSP usually needs more supporting I/O chips, it has the potential of
realizing high performance system even compared with analog control. There is also a
tendency of integrating more I/O functions into DSPs.
As mentioned earlier, there are some major limitations for digital controller implementation of
power converters. The first is output inaccuracy due to the finite wordlength of the ADCs, and
DACs or PWM outputs. The second is achievable system bandwidth associated with the
sampling and computing delays. The third one is the overall functional integration, which is
closely related to the overall system structure and cost.
18
So for practical purposes, DSP-based system can achieve real time PWM control, even cycle-
by-cycle control with satisfactory performance (as shown in Figure 1.5). While
microcontroller-based systems have to rely on additional PWM control chips to accomplish
the PWM switching function (for a few hundreds of kilohertz) and as an interface a digital- to-
analog converter (DAC) is needed here (as shown in Figure 1.4) [12]. The PWM control is far
from cycle-by-cycle. The control output from the controller will be updated at a relatively low
speed compared to switching frequency and it is a sample and hold process in principle.
So from speed and accuracy point of view, DSP can achieve real-time PWM control up to a
few hundreds of kilohertz without additional DAC and PWM control chip and microcontroller
cannot and have to rely on additional DAC and PWM control chip for present-day power
supplies. It should be noted that achievable bandwidth really depends on the complexity of the
control algorithm and the capacity of the instruction set. We only consider the case of single
output voltage regulation. It is also possible to realize parallel operation of multiple
converters, e.g. VRMs, with more advanced control strategy. Finally, we have to point out
that even DSP-based system cannot realize peak current mode control due to the excessive
speed requirement of the current sensing though average current mode control is possible.
Figure 1.4 Typical Microcontroller-Based Digital Control System Diagram for PWM DC/DC Converter
Interface circuit
gain
PWM Stage
gain
DAC
Source
Power
Converter
Load
V
con
v and/or i
d
i
sw
Microcontroller
ADC
ADC
Digital
Controller
19
Figure 1.5 Typical DSP-Based Digital Control System Diagram for PWM DC/DC Converter
1.2.2 Digital PWM Control Using DSP
PWM Controllers
Pulse Width Modulation controllers are implemented using both analog and digital control
schemes. Pulse width modulator produces a logic signal, which is periodic with frequency f
and has duty cycle d. The signal is used to control the duration over which power transistor in
the converter are switched on. The input to the pulse width modulator is an analog control
signal. The modulator manipulates the analog control voltage to produce the duty cycle in
proportion to it.
With the recent advances in the semi-conductor industry, microprocessors and digital signal
processors are used in producing digital PWM. A PWM controller using DSP is shown in
Figure 1.6 [13]. In this control scheme the parameters of interest like the output voltage, input
voltage and current are fed into a set of preamplifiers and digitized using an A/D converter.
These parameters are manipulated suitably in the digital signal processor where the required
numerical values namely Ts, Ton, Ts, corresponding to switching period, on-time for the
converter switches and start time for the A/D signal conversion are generated. The
Interface circuit
gain
PWM Stage
gain
Source
Power
Converter
Load
v and/or i
d
DSP
ADC
ADC
Digital
Controller
20
calculations of these values are based on a set of gain equations. The architecture of this
approach is shown in Figure 1.6.
Figure 1.6 Architecture of Digital PWM Using Digital Signal Processor
OSC
Gate drive
for Q1
Gate drive
for Q2
Parameters
V
in
I
in
V
0
Drive signal generator
NT
s
NT
on1
NT
on2
NT
d
Pre-amplifier
circuit
Digital Signal Processor
(DSP)
A/D signal
converter
T
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T
on
T
d
CK1
f
clk
NV
in
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21
1.3 Digital Signal Processors
A Digital Signal Processor is a super-fast chip computer, which has been optimized for the
detection, processing and generation of real world signals such as voice, video, music, etc, in
real time. It is usually implemented in a single chip or nowadays just part of an IC, about 0.5
cm² to 4 cm² [14].
In contrast, a microprocessor is traditionally a much less powerful computer that performs the
mundane tasks, often controlling other devices e.g. keyboard entry, central heating, washing
machine cycles, etc.
Digital signal processors were created many years after general-purpose processors. The need
for digital signal processors was due to specific features of digital signal processing
algorithms. These algorithms usually include MAC (multiplication and accumulation), where
each item of a MAC is in turn a multiplication of other two items. Hence, the main operation
in algorithms of digital signal processing is the combination of multiplication and
accumulation.
Nearly every DSP is capable of performing a so-called MAC operation in one instruction
cycle. The operation consists of a multiply and a subsequent addition (accumulation), as
depicted in Figure 1.7.
22
Figure 1.7 MAC Operation
The rapid development of microelectronics makes the manufacturing of very fast digital
signal processors easier all the time and also makes it possible to perform more demanding
signal processing operations digitally. When compared to general purpose processors, digital
signal processors are characterized by the capability of performing the mathematical
operations of digital signal processing (DSP) very fast.
The central problem of digital signal processing is to enable systems to work in real time. This
means that it is necessary to complete all operations of a signal processing algorithm within
the discretization period of this signal. To reach high efficiency in computations mainly based
on multiply and accumulate operations in general purpose processors is a difficult task.
Therefore, the new type of processors digital signal processors, which provide an easy and
efficient way of realization of DSP algorithms in real time, was offered on market in the early
80s.
DSP is used in all kinds of applications that require digital manipulation of information or
digital system control. DSP is especially important to digital cellular telephony and
teletechnology in general.
Operand1 Operand2
Rounding
Operand1
Scaling
23
Figure 1.8 In DSPs, an analog signal such as voice is digitized by an analog-to-digital converter (A/D). The DSP
processes the digital signal, then a digital-to-analog converter (D/A) changes the signal back to
analog [15].
What is actually the difference between digital signal processors and general-purpose
processors? To reach the necessary speed in digital signal processors, the main hardwired
operation of DSP is the multiply-accumulate operation, which can be performed within one
clock cycle. From the architectural point of view digital signal processors use different, so-
called modified Harvard processor architecture that enables command pipelining (processing
parts of commands in parallel). Memory is divided into two parts: data memory and
instruction memory. It is possible to move data directly between these two types of memory.
This approach made possible to organize efficient parallelization of calculations, i.e. "fetch,
decode, execute" phases of adjacent instructions can be performed in parallel.
Figure 1.9 Harvard Architecture [16]
CPU/ALU
Program
Memory
P Address
Instructions
Data
Memory
#2
Data
Memory
#1
D1 Address
D1 Data
D2 Address
D2 Data
When you speak,
your voice is picked
up by an analog
sensor in the cell
phone's microphone.
An analog-to-digital
converter chip
converts your voice,
which is an analog
signal, into digital
signals, represented
by 1s and 0s.
The DSP compresses
the digital signals and
removes any
background noise.
In the listener's cell
phone, a digital-to-
analog converter chip
changes the digital
signals back to an
analog voice signal.
Your voice exits
the phone
through the
speaker.
24
CPU/ALU
Memory
Address
Data
Conventional microprocessors use the Von Neumann architecture: program and data all in a
single memory. Address and data buses are shared between instruction and data fetches [16].
Figure 1.10 Von Neumann Architecture
While digital signal processors are well suitable for signal processing, general-purpose
processors are better on general computing applications. Desktop general-purpose processors
have hardware support for floating point and integer arithmetic.
Often digital signal processors are divided into two types, floating-point processors and fixed-
point processors, depending on the type of arithmetic used in the processor for calculations.
Digital signal processors can also be divided into algorithm-specific and application-specific
processors. Desktop general-purpose processors must be able to support code written for
previous designs of these processors. They must support object-code compatibility. In case of
digital signal processors it is not so critical. Digital signal processors and general-purpose
processors also differ in memory organization principles. For general-purpose processors it is
traditional to use on-chip instruction and data caches as their only on-chip RAM. They also
support off-chip memory and second-level caches. Digital signal processors usually have on-
chip RAM and very seldom use caches.
Usually most digital signal processors share several common features to support high-
performance, repetitive, numerically intensive tasks. As already mentioned above, one of the
central operations of most digital signal processors is the multiply and accumulate operation
(MAC). All digital signal processors have the ability to perform multiply-accumulate
operations in a single instruction cycle. To support such a feature, digital signal processors
include accumulator and multiplier integrated into a special arithmetic processing unit in the
data path of the processor. Some modern digital signal processors provide two or even more
multiply-accumulate units. This feature allows multiply-accumulate operations to be
performed in parallel. Generally digital signal processors provide extra "guard" bits in the
accumulator to allow a series of multiply-accumulate operations to proceed without causing
25
arithmetic overflow (when numbers exceed the maximum value the processor's accumulator
can hold).
A beneficial feature shared by digital signal processors is called multiple-access memory
architecture. This means that it is common for most digital signal processors to provide the
ability to complete several accesses to memory in a single instruction cycle. This feature
allows the processor to fetch an instruction while simultaneously fetching operands and
storing the result of a previous instruction to memory. For instance, for computing the vector
dot product for an FIR filter, most digital signal processors are able to perform a MAC while
simultaneously loading the data sample and coefficient for the next MAC. But on the other
hand such single-cycle multiple memory accesses are often subject to some limitations.
Usually, all but one of the memory locations accessed must reside on-chip, and multiple
memory accesses can only take place with certain instructions. Digital signal processors
provide multi-ported on-chip memories, multiple on-chip buses, and sometimes multiple
independent memory banks to support simultaneous access of multiple memory locations.
Another feature often used to speed up arithmetic processing on digital signal processors is
one or more dedicated address-generating units. This feature enables performing address
calculation for operand access in parallel with the execution of arithmetic instructions.
General-purpose processors usually require additional cycles to generate the addresses needed
to load operands. Address generation units of digital signal processors usually support several
addressing modes. These modes are register-indirect addressing with post increment/
decrement (for example, this mode is widely used during repetitive computations on data
stored sequentially in memory). Another mode, called modulo addressing, is often supported
to simplify the use of circular buffers. Some processors also support bit-reversed addressing,
which increases the speed of fast Fourier (FFT) algorithms.
Many DSP algorithms involve performing repetitive computations. Because of this fact, most
DSP processors provide special support for efficient looping. Special repeat or loop
instruction is provided, allowing implementation of a for-next loop without spending extra
instruction cycles for updating and testing the loop counter or branching back to the top of the
loop. It is called zero-overhead looping.
26
An additional feature shared by digital signal processors is the ability to allow low-cost, high-
performance input and output. Most digital signal processors incorporate one or more serial or
parallel I/O interfaces, and specialized I/O handling mechanisms such as low-overhead
interrupts and direct memory access (DMA), which allow data transfers to proceed with little
or no intervention from the rest of the processor [17].
There are a lot of DSP algorithms and many system applications use it. Digital signal
processors find wide usage in speech coding/decoding, modem algorithms, image
compression spectral estimation, hi-fi audio and so on. We do not describe the details of the
general purpose digital signal processor architecture here, because one of the following
chapters contains a description of TMSC320F2812 DSP developed by Texas Instruments,
which has been used for the purposes of this work.
1.3.1 Data Path of a DSP
The data path of a digital signal processor is where the vital arithmetic manipulations of
signals take place. DSP data paths are highly specialized to achieve high performance on the
types of computation most common in DSP applications, such as multiply-accumulate
operations. Registers, adders, multipliers, comparators, logic operators, multiplexers, and
buffers represent 95% of a typical DSP data path.
Multiplier
A single-cycle multiplier is the essence of a DSP since multiplication is an essential operation
in all DSP applications. An important distinction between multipliers in DSPs is the size of
the product according to the size of the operands. In general, multiplying two n-bit fixed-point
numbers requires a 2xn bits to represent the correct result. For this reason DSPs have in
general a multiplier, which is twice the word length of the native operands.
Accumulators & Registers
Accumulators and registers hold intermediate and final results of multiply-accumulate and
other arithmetic operations. Most DSP processors have two or more accumulators. In general,
the size of the accumulator is larger than the size of the result of a product. These additional
27
bits are called guard bits. These bits allow accumulating values without the risk of overflow
and without rescaling. N additional bits allow up to 2
n
accumulations to be performed without
overflow. Guard bits method is more advantageous than scaling the multiplier product since it
allows the maximum precision to be retained in intermediate steps of computations.
ALU
Arithmetic logic units implement basic arithmetic and logical operations. Operations such as
addition and subtraction are performed in the ALU.
Shifter
In fixed-point arithmetic, multiplications and accumulations often induce a growth in the bit
width of results. Scaling is then necessary to pass results from stage to stage and is performed
through the use of shifters.
1.3.2 Peripherals of a DSP
Most digital signal processors provide on-chip peripherals and interfaces to allow the DSP to
be used in an embedded system with a minimum amount of external hardware to support its
operation and interfacing.
Serial Port
A serial interface transmits and receives data one bit at a time. These ports have a variety of
applications like sending and receiving data samples to and from A/D and D/A converters and
codecs, sending and receiving data to and from other microprocessors or DSPs,
communicating with other hardware. The two main categories are synchronous and
asynchronous interface. The synchronous serial ports transmit a bit clock signal in addition to
the serial bits. The receiver uses this clock to decide when to sample received data. On the
opposite, asynchronous serial interfaces do not transmit a separate clock signal; they rely on
the receiver deducing a clock signal from the data itself.
28
Direct extension of serial interfaces leads to parallel ports where data are transmitted in
parallel instead of sequentially. Faster communication is obtained through costly additional
pins.
Host Port
Some DSPs provide a host port for connection to a general-purpose processor or another DSP.
Host ports are usually specialized 8 or 16 bit bi-directional parallel ports that can be used to
transfer data between the DSP and the host processor.
Link Port or Communication Port
This kind of port is dedicated to multiprocessor operations. It is in general a parallel port
intended for communication between the same types of DSPs.
Interrupt Controller
An interrupt is an external event that causes the processor to stop executing its current
program and branch to a special block of code called an interrupt service routine. Typically
this code deals with the origin of the interrupt and then returns from the interrupt. There are
different interrupt sources: on-chip peripherals: serial ports, timers, DMA,...
External interrupt lines: dedicated pins on the chip to be asserted by external circuitry
Software interrupts: also called exceptions or traps, these interrupts are generated under
software control or occurs for example for floating-point exceptions (division-by-zero,
overflow and so on).
DSPs associate interrupts with different memory locations. These locations are called
interrupt vectors. These vectors contain the address of the interrupt routines. When an
interrupt occurs, the following scenario is often encountered:
·
Save program counter in a stack
·
Branch to the relevant address given by the interrupt vector table
·
Save all registers used in the interrupt routine
·
Perform dedicated operations
29
·
Restore all registers
·
Restore program counter
Priority levels can be assigned to the different interrupt through the use of dedicated registers.
An interrupt is acknowledged when its priority level is strictly higher that current priority
level.
Timers
Programmable timers are often used as a source of periodic interrupts. Completely software-
controlled to activate specific tasks at chosen times. It is generally a counter that is preloaded
with a desired value and decremented on clock cycles. When zero is reached, an interrupt is
issued.
DMA
Direct Memory Access is a technique whereby data can be transferred to or from the
processor's memory without the involvement of the processor itself. DMA is commonly used
to provide improved performance with input/output devices. Rather than have the processor
read data from an I/O device and copy the data into memory or vice versa, a separate DMA
controller can handle such transfers in parallel.
30
1.4 Digital Signal Processing
To `digital signal process' is to manipulate signals that have either originated in, or are to be
exported to, the real world, where those signals are represented as digits (numbers) [14].
Application of digital processing devices to transmit, store and process analog signals require
their representation in units of discrete points as numerical values. This means that an integral
part of DSP applications is the conversion of the real world signals analog voice, music,
video, engine speed, ground vibration to numerical values for processing by the DSP a
process termed analog to digital conversion (A/D). Discretization can be performed by
analog-to-digital converter (ADC) at the input of DSP system. Going in the other direction
the conversion of numerical values generated within the DSP to real world signal a process
termed digital to analog conversion (D/A) is also involved. Restoration of signal back to
analog form is made via digital-to-analog converter (DAC) at the output. To digitally
represented signals different mathematical operations can be applied and that is the essence of
digital signal processing.
Digital signal processing is used in a wide range of applications, in which the nature of
processed signals can be very different radio, sound, video etc. Fundamental to DSP is the
fact that any real world signal, e.g. music, can be accurately represented by samples of the
signal taken at periodic intervals. These samples can then be converted into numbers (e.g.
representing the volume of the music at the sample point), and these numbers are expressed in
binary form. The level of processing complexity can vary depending on application specifics
and performance requirements.
Why did digital signal processing become so popular? The answer is in fact that digital signal
processing has several advantages compared to analog signal processing. DSP systems are
able to complete the tasks inexpensively so to say. Especially it refers to control part of
applications. It is much more difficult to control analog signals processing. First of all, analog
systems are more sensitive to environment conditions. For example, analog circuit
performance can depend a lot on its temperature and change behavior according to changes in
its temperature. Analog components have certain tolerances in their values. That means that
two circuits of the same design can respond differently having little differences in their
internal analog components. In contrast, two digital systems with identical functionality will
31
produce the same results on the same input. This makes digital systems more predictable and
reliable than analog.
DSP systems are reprogrammable. One DSP system can be programmed to perform different
tasks. If you want to change functionality on analog system - other physical components
should be installed. Analog components tend to have different size for different values. For
example to increase capacitance, bigger capacitor should be used. In DSP processing you
would need only to increase the value in the program, but the hardware part stays unchanged.
Depending on class of applications different aspects are important. The largest class of DSP
applications is inexpensive high-volume embedded systems, such as modems, cellular
telephones, pagers and so on. Here, cost and power consumption are factors of main
importance. Another class is processing large amounts of data using complex algorithms, like
in sonars, radars and seismic exploration. Performance and multiprocessor support are
favorites among aspects for these applications. A separate category consists of Personal
Computer (PC) -based multimedia applications. It becomes very popular to integrate DSP
systems into PCs to perform multimedia functions, like voice mail or music synthesis
Performance is also important for these devices because the DSP part can be asked to execute
several algorithms simultaneously. Memory capacity may be an important issue to consider in
these applications too [17].
1.4.1 The History of DSP
Since the invention of the transistor and integrated circuit, digital signal processing functions
have been implemented on many hardware platforms ranging from special-purpose
architectures to general-purpose computers. DSP as a device, a process or a subject at
University is relatively new. In the 1960s, DSP hardware used discrete components and
consequently, because of the high cost and volume, its application could only be justified for
very specialized requirements. In the 1970s, monolithic components for some of the DSP
subsystems appeared, primarily dedicated digital multipliers and address generators, and DSP
systems could be implemented using bit slice microprocessors. The breakthrough for mass
exploitation of DSP techniques came in 1979 when Intel introduced the 2920, a completely
self-contained signal processing device in a 40-pin DIP package incorporating on-board
program EPROM, data RAM, A/D and D/A converters, and an architecture and instruction set
32
powerful enough to implement a full duplex 1200 bps modem, including transmit and receive
filters. This breakthrough was followed up convincingly by Texas Instruments in 1982 with
the launch of the TMS32010.
Since the Intel 2920 there have been five further generations of general-purpose signal
processing devices, with a sixth generation announced. The latest devices have some 100000
times the processing power of that early 2920 device, all within the space of 20 years.
The following table summarizes the evolution of DSPs:
Date
Features
Example processors
First generation :
1979 - 1985
Harvard architecture,
hardwired multiplier
NECPD7720, Intel 2920, Bell
Labs DSP1, TMS320C10
Second generation: 1985
- 1988
Concurrency, multiple
busses, on-chip memory
TMS320C25, MC56001, DSP16
(AT&T)
Third generation:
1988 - 1992
On-chip floating point
operations
TMS320C30, MC96002, DSP32C
(AT&T),
Fourth generation:
1992 - 1997
Multi-processing features
Image and video processors
Low-power DSPs (AT&T)
TMS320C40&50, TMS320C80
Fifth generation: 1997
VLIW
TMS320C6x, Philips TriMedia,
Motorola Starcore
Table 1.1 Evolution of DSPs.
Today, all of the major semiconductor device manufacturers have DSP products either already
released or under development, reflecting their confidence in an enormous growth of demand
in the late 1980s, 1990s and now into the 21
st
century, paralleling the spectacular success of
microprocessors in the 1970s.
Digital signal processing in general and DSP processors in particular, are used in a wide
variety of applications from military radar systems to consumer electronics. Naturally, no one
processor can meet the needs of all applications. Criteria such as performance, cost,
integration, ease of development, power consumptions are key points to examine when
designing or selecting a particular DSP for a class of applications.
Speech analysis was the driving force behind initial attempts to process signals digitally. The
exacting tolerances demanded of filters in speech processing systems simply could not be
maintained over time with analog techniques, subject as they are to temperature drift,
33
component tolerances, and aging. Digital processing operations, in contrast, consisting of
nothing more than sequences of binary multiplication, addition, subtraction, etc, make the
outcome entirely predictable and thus reproducible. This feature, together with the ability to
replicate multiple analog processing tasks within a single low-power device, is the reason why
DSP is a thriving multi-billion dollar industry [14].
Details
- Pages
- Type of Edition
- Erstausgabe
- Year
- 2014
- ISBN (eBook)
- 9783954897360
- ISBN (Softcover)
- 9783954892365
- File size
- 3.8 MB
- Language
- English
- Publication date
- 2014 (March)
- Keywords
- Digital Signal Processor DC-DC Resonant Converters Power Electronics Pulse Width Modulation C++
