Digital Signal Processors for Real-Time Embedded Systems

1. Digital Signal Processors for Real-Time Embedded Systems By Jeremy Kohel

2. Overview • What is a DSP? • Common characteristics of DSP’s • Software considerations • Available tools • Example code

3. 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?

4. Digital Signal Processing in Action

5. Common characteristics of DSP’s • The ability to perform many highly numeric intensive tasks at fast speeds • Efficient instructions (MAC’s) • Efficient memory access • Efficient address generation

6. MAC’s • MAC’s = 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

7. MAC’s (cont.) • MAC’s are helpful to calculate the sum of many multiplication operations • Vector dot products used in many algorithms, i.e. filtering • Some DSP’s have multiple accumulators and multiplier units so they can perform many MAC’s in a single instruction cycle

8. Basic DSP architecture

9. MAC example

10. Efficient Memory Access • DSP’s 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

11. 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

12. Address Generation • DSP’s 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

13. 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.

14. 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

15. 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

16. 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 • Doesn’t waste cycles on checking count variables or branching back to the top of a loop

17. 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

18. More Disadvantages • DSP data is stored differently (No integers) • Only fractional part of floating point numbers • Therefore you can’t get a number greater than 1 • $7FFFFF = 0.99999988709 • Highest number allowed • MSB is the sign bit

19. 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)

20. Acc (cont.) • If the number stored is 24 bits then its stored in A1 with A0 being zeroed out and A2 being sign extended • $FF:834345:000000 • Otherwise the MSB of A2 is the sign bit with the other 7 being the integer and the other 48 being the decimal • $00:834345:125345 (Allows for numbers between –128.0 and 127.99)

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

22. 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

23. 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)

25. 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

26. DirectDSP

27. Code Composer Studio • Distributed by Texas Instruments • Provides a user-friendly IDE • C/C++ compiler • Project Manager • Simulator

28. 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

29. 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

30. Problems with code generation • Doesn’t provide optimized code • Must be hand optimized • Won’t generate assembly libraries

31. 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;

32. More code for (n=0; n<nLen; n++) {  x[n] = 0.75*x[n] + 1000; y[n] = x[n]; } }

33. Conclusion – Things to remember • DSP’s 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

34. References • [1] “Choosing a DSP Processor”, Berkeley Design Technology white paper, http://www.bdti.com/articles/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?page=ccode#using • [4] "DSP Algorithm Development Tools" DSP & Multimedia Technology, November 1993

35. References (cont.) • [5] “Digital Signal Processing, C6000 DSPs”, Texas Instruments, http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=dsp&sectionId=2&tabId=57&familyId=132 • [6] “C-Language Programming for DSP”, Pentek Inc white paper, http://www.pentek.com/deliver/TechDoc.cfm/C_LangProg.pdf?Filename=C_LangProg.pdf