Digital Signal Processors for Real-Time Embedded Systems

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Digital Signal Processors for Real-Time Embedded
Systems

• By Jeremy Kohel

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Overview

• What is a DSP?
• Common characteristics of DSPs
• Software considerations
• Available tools
• Example code

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What is a DSP?

• Definition
• A specialized microprocessor designed
  specifically for the rapid processing of digital
  signals in real time.
• What does this mean?
• Where are they found?

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Digital Signal Processing in Action
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Common characteristics of DSPs

• The ability to perform many highly numeric
  intensive tasks at fast speeds
• Efficient instructions (MACs)
• Efficient memory access
• Efficient address generation

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MACs

• MACs multiply-accumulate instructions
• Highly used in DSP applications
• Must be done in at most a single instruction
  cycle
• Equivalent to a a (b x c) where a is an
  accumulator register

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MACs (cont.)

• MACs are helpful to calculate the sum of many
  multiplication operations
• Vector dot products used in many algorithms, i.e.
  filtering
• Some DSPs have multiple accumulators and
  multiplier units so they can perform many MACs
  in a single instruction cycle

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Basic DSP architecture
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MAC example
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Efficient Memory Access

• DSPs must be able to make multiple access to
  memory in a single instruction cycle
• Allows for fetching the next instruction while at
  the same time fetching operands and/or saving
  data to memory from a previous instruction

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In order for this to happen

• Requires multiple on-chip buses
• Requires multiple on-chip memory banks
• Only a few instructions in the entire instruction
  set have this ability

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Address Generation

• DSPs require separate generation units in order
  to find the next address needed
• Run in the background outside the main data path
• Allows an address of operand access to be
  calculated at the same time as performing
  arithmetic operations

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Software Considerations

• How will the program be developed/tested
• DSP hardware, simulator
• In what language will the program be developed?
• Assembly, C/C, Ada, etc.

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Cross Compiler vs. Native Compiler

• DSP object code differs from the object code of a
  regular, CPU-driven workstation
• Cross compiler runs on CPU workstation and
  creates DSP object code
• Native compiler runs on CPU workstation and
  creates object code for that computer
• Either have DSP hardware at hand or test with a
  simulator to mimic hardware

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Language Dilemma

• Assembly vs. C
• Compilers are available for other higher level
  languages
• C is most commonly used in real-time applications
• Advantages and disadvantages of each

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DSP Assembly

• More efficient than C code
• Provides a more optimal solution
• Includes instructions to make common tasks more
  efficiently executed
• Loop or Repeat instruction
• Doesnt waste cycles on checking count variables
  or branching back to the top of a loop

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Disadvantages of DSP Assembly

• Takes longer to write code
• More difficult to write solutions
• R7 Max(R5, R6)
• LDF R7, R6
• COMF R5, R7
• LDFLT R7, R5

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More Disadvantages

• DSP data is stored differently (No integers)
• Only fractional part of floating point numbers
• Therefore you cant get a number greater than 1
• 7FFFFF 0.99999988709
• Highest number allowed
• MSB is the sign bit

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Problems?

• There can be numbers larger than 1 or smaller
  than 1
• This is allowed using the accumulator and 56 bits
• Accumulator A2(8 bits)A1(24 bits)A0(24 bits)

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Acc (cont.)

• If the number stored is 24 bits then its stored
  in A1 with A0 being zeroed out and A2 being sign
  extended
• FF834345000000
• Otherwise the MSB of A2 is the sign bit with the
  other 7 being the integer and the other 48 being
  the decimal
• 00834345125345
• (Allows for numbers between 128.0 and 127.99)

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Embedded C

• Easier to write code for
• Programs are shorter and less complex
• Compilers available
• C is very versatile, and highly portable

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Problems with C

• The resulting assembly code is not optimal and
  therefore must be hand optimized
• Many compilers claim code optimized for density
  and execution time
• Most of the time this is not the case
• Compilers allow for assembly code to be inserted
  inline

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Other issues

• C requires a large number of external libraries
  that need to be compiled into the program
• I/O libraries
• Run-Time libraries (math, string, memory)
• DSP libraries (matrix arithmetic, filtering,
  image processing)

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Software Tools

• DirectDSP by Signalogic
• Creates an interface between the DSP hardware and
  other well known development environments
• Matlab, Visual Studio, .NET
• Real-time watches
• Waveform generator

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DirectDSP
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Code Composer Studio

• Distributed by Texas Instruments
• Provides a user-friendly IDE
• C/C compiler
• Project Manager
• Simulator

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CCS

• Real-time analysis (similar to DirectDSP)
• Cache log
• Color codes cache hits to optimize algorithm
  placement
• Code Coverage
• Highlights lines of code not executed
• Lists number of times lines are executed
• Allows optimization

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ANSI C Code Generator

• Distributed by Hyperception
• Works in coordination with graphical design
  environments
• Creates C code based on the design
• Good for porting algorithms
• Decrease develop time

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Problems with code generation

• Doesnt provide optimized code
• Must be hand optimized
• Wont generate assembly libraries

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Code Example

• void UserProc(void ptrIn, void ptrOut, long
  nLen, short int nNumTrace)
• ifdef defined(TMS320C3x) defined(TMS320C4x)
  defined(DSP5600x) defined(ADSP2106x)
  
• define x ((long)ptrIn)
• define y ((long)ptrOut)
• endif
• short int n

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More code

• for (n0 nltnLen n)
•  
• xn 0.75xn 1000
• yn xn

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Conclusion Things to remember

• DSPs are not versatile, they are specialized to
  a specific task
• CPU/DSP systems popular
• They value efficiency over oscillator frequency
• When developing a DSP application higher level
  languages are easier, but will not give as
  optimized code as assembly

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References

• 1 Choosing a DSP Processor, Berkeley Design
  Technology white paper, http//www.bdti.com/articl
  es/choose_2000.pdf
•  
• 2 Jennifer Eyre and Jeff Bier, The Evolution
  of DSP Processors, Berkeley Design Technology
  white paper
• http//www.bdti.com/articles/evolution.pdf
• 3 Adding user defined C routines to real-time
  DSP code, Signal Logic, http//www.signalogic.com
  /index.pl?pageccodeusing
• 4 "DSP Algorithm Development Tools" DSP
  Multimedia Technology, November 1993

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References (cont.)

• 5 Digital Signal Processing, C6000 DSPs,
  Texas Instruments, http//focus.ti.com/paramsearch
  /docs/parametricsearch.tsp?familydspsectionId2
  tabId57familyId132
•  
• 6 C-Language Programming for DSP, Pentek Inc
  white paper, http//www.pentek.com/deliver/TechDoc
  .cfm/C_LangProg.pdf?FilenameC_LangProg.pdf