DSP of Any Kind

DSP of Any kind : boards, FPGA,  Custom chips etc..
The goal of this section is to cover the "free world" of DSP architecture.

By definition, commercial GP (General Purpose) DSPs such as a TI C64xx are not free. They must follow the constraints of Chip design, ISA design and most of all, compiler friendly and software upward compatibility.
By comparison, there are many products which are point products or application specific. They are full of interesting tricks and original solutions.This is the case for custom DSP chips, and they can also be found among boards and FPGA based platforms.

The problem with all these custom solutions is the lack of public information.

• FPGA is the most obscure because the description is both code (HDL) and proprietary. 
• Custom chips are more available in the form of conference papers [ref 1]. 
• Boards, especially older boards, are the most accessible since the description is a block diagram  

Background

From 1978 to 1985, designing dsp boards with dsp BB
A big application of bit slice families were DSP boards. We largely used the architecture of these boards to design the first generation of integrated GP DSPs.
From 1985 to 1995, designing dsp boards with GP DSP
Once integrated GP DSPs were available, the board architecture became predictable and most of the advances were largely outside DSP such as host interfacing[ref 10..13]
Since 1995, FPGA

Around 1995 was the time that , {with a different type of footprint}, the FPGA  became a credible alternative to board.  Not surprisingly the FPGA companies reinvented many of the BB and board architecture techniques of the 80s.

DSP BOARDS                                                                           

The historical path to electronics progress is to design boards based both on new components and better architecture understanding. And new components are then developed by integrating what was then a full board of electronics. Recent footprint changes (SIP) and nanotechnology did not fundamentally alter the process. A end-user board still represents a very good approximation of the next generation SOC platform. So in a methodology like ours, which design COPS by relying on proven DSP architectures, studying DSP boards will represent some design values.
Let us start by categorizing DSP boards and keep in mind that the same categories could be applied to COPs.:

1. Boards designed using  a bit slice family (AMD) or a building block family (ADI).
2. Boards based on a General Purpose DSP (typically a TI DSP ) or high performance CPU.
3. Special purpose boards such as Array processors or Vector Processors.
4. Boards based on one or several custom chips  

FPGA

In a way, FPGA can be seen as a new type of board. There are however deep differences. 

• The FPGA vendors (Xilinx, Altera, Quicklogic) propose catalogs of Intellectual Properties (IP) which constitute an important reference for the COP designer.
• They also propose multiple builder tools. Some can serve as example for developing COP.
• Finally, they have multiple experiences with "Matlab" to implementation (in the form of Simulink block to Xilinx hard macro). 

Note: we do not try to  cover the implementation of a COP in a FPGA fabric. Instead we imply that the FPGA ecosytem constitutes a vast amount of resources to be tapped in.

DSP CUSTOM CHIPS

A long time ago, custom chips were a mine of information. For instance the key search of  'FFT' on the IEEE ISSCC DVD will bring in the region of 100 hits over the 1980s 10 years span. Nowadays the number of custom chips has dramatically diminished and the descriptions are rarely at the level of the building block. Still like boards they are a very good reference for communication and multiple processing.

Also we should not forget that a COP is a form of custom chip.

FURTHER: WHERE TO START WITH DSP BOARDS?

Motivation: "should a DSP COP architect be studying DSP boards?".
1) Today's DSP boards: they are too sophisticated to serve as good examples. Their design advances have more to do with inter-board techniques and software than "pure"DSP.  Still there are few techniques worth studying such as HOST-COP interfacing, serial inter-chip communication (SRIO) and heterogeneous MP.
2) DSP boards of the 70-80s: they are much more amenable to study. They had a large range of architectures and structures [ref 2-9]. Theys are nearer to the architecture of a current COP design than current DSP boards and chips. 

• One could think "If the DSP chip architects had done a good job, we would not need to go back to obscure stuff to get our architecture inputs". But this would be unfair since designing a GP DSP had  many constraints given by software. We will not say that these constraints should be totally ignored by the COP designer but the whole point of designing a COP is for these constraints to take a back seat.   

We will now do a quick review of some paper references and draw conclusions on what can be possibly reused:

1. Most articles had a block diagram of (what was supposed to be) a DSP
2. The definition of a DSP varied widely. For example:
1. Usage of serial arithmetic.   
2. Usage of shift register as opposed to memory
3. A memory model reduced to I/O buffers (ADC/DAC) so that there was no need of AGU.
1. Even a model with only DAC (MIT speech generator)
4. AGU with flags going back to the ALU status registers. Obviously to do circular.
5. Single bus architecture, and the Zurich zip trick as an architecture feature 
3. Veendrick [ref. 11]  had an interesting BB that we will call "MULIMIT"
1. Z= K*A + L*B

References                                                                                                                                      

1. Hot chips, ICSPAT, ICCASP, ISSCC, see also IEEE DVDs (Com. Soc., SP Soc. SSC. Soc.)
2. Louis Schirm IV, TRW inc. "Packing a signal processor on a single digital board" electronics dec 20, 1979
3. John Mick "Fast computational devices  for DSP" likely at some conference before 1980 (?)
4.  Zaheer Ali "Know the LSI hardware of digital signal processors" EDN 21 june, 1979
5. Capello, etc.. "Completly pipelined architectures for DSP" IEEE Vol ASSP-31, August 1983
6. S.Chin, C.Brooks "Microprogramming enhances signal processor's performance"   Electronics, nov 17, 1982
7. Zemam, Troy Nagle " A high speed microprogrammable DSP employing distributed arithmetic" IEEE Vol SC-15, Feb 1980
8. R.Shively "Architecture of a programmable DSP" IEEE Vol C-31, Jan 1983
9. IBM SP16
10. MIT Klatt vocal tract model" ICCASP 82 ??
11. Phillips Labs "A 40 MHz Multiapplicable DSP chip" IEEE Vol SC-17, Feb 1982
12. TRW ICCASP 83
13. David Karlin ( Fairchild) " VLSI BB for DSP" ICCASP 1982
14. Barral, Moreau "Circuits for Digital signal processing" ICCASP 1984
15. NTT "LSI's for DSP" IEEE vol. SC-14, April 1979
16. F. Mintzer, A.Peled "The architecture of the real-time signal processor "ICCASP 1982
17. and dozens of others with key searches in IEEE explore
18.  Plug-in DSP boards" EDN April 26, 1990
19. "DSP coprocessor boards" EDN Sep 13, 1991
20. "DSP boards help tackle a tough class of AI tasks" electronics, aug 21,1986
21. G.Pawle, T.Faherty "DSP development board offers host independence" computer design october15,1984