Add a little DSP…

Add a little DSP…The benefits of adding DSP functionality to embedded microprocessor architectures are such that it is surely only a matter of time before designers change from the traditional route. Richard Wilson reports
A couple of months back executives from microprocessor firm ARMtold a room full of City analysts that it was setting its sights on a potentially explosive new market. Automotive, or putting processors in cars and trucks, was that market. The City types were impressed and ARM’s share price continued its upward climb.
For the engineering community one of the main jig-saw pieces behind that strategy slotted into place a couple of weeks ago with the announcement at the Embedded Processor Forum in California of new digital signal processing (DSP)extensions for the ARM9 processor core. Just like high-tech share prices, the trend of adding DSPfunctionality to embedded microprocessor architectures is certainly on the upward curve. Shopping basket
The ARM9E , a 32-bit RISC core with DSP capabilities is an extension of the ARM9TDMI core with 32 x 16 and 16 x 16 multiply instructions that offer efficient use of a 32-bit bandwidth for packed 16-bit operands.
Lexra’s MIPS-based core, the LX5280, incorporates a set of 36 DSP extensions to the MIPS instruction sets.
ARC Cores’ version 3.0 ARC core includes DSP extensions such as configurable multiply-accumulate (MAC) capability and XY memory, saturating add and subtract, instruction cache options, and DSP library support.
STMicroelectronics has the ST100 a 32-bit Risc DSP core which provides up to 600 million MAC operations. A 64-bit ST1000 core planned for late next year could crank that up to 1.6 billion MAC operations.
Texas Instruments’ TMS320C27x core architecture announced last year was the company’s first to combine DSP and microprocessor capabilities in a single building block.
Infineon Technology plans to have by the middle of the year first microcontrollers based on its TriCore DSPhybrid, targeting automotive applications and computer peripherals such as hard disk drives.
Hitachi has identified GSM and UMTS mobile phone design as a target applications for its Risc/DSP microprocessor the SH-DSP first launched last year.  
 
For years the development of microprocessor and Harvard DSP architectures went their separate ways. Even if you wanted to combine the two functions you couldn’t without considerable difficulty.
It is only relatively recently with the introduction of “Harvard-like” architectures in microprocessor cores such as ARM,ARC and Lexra, all three announced micros with DSPfunctionality at the Embedded Processor Forum, that the combination became a practical option.
And the argument is a strong one. Designers maintain that by combining the microprocessor and DSP hardware elements you can make the most efficient use of die area.
It is a fact that DSP heavy-weights Lucent Technologies and TexasInstruments have climbed to the top of the world’s top five ranking of embedded processor suppliers, according to Dataquest. This has prompted some pundits to jump on the bandwagon claiming that a good DSP core is now more important to a supplier than a microprocessor core for success in the embedded market.
This is clearly nonsense, but it cannot be denied that the hybrid micro/DSPproduct is an attractive design option for a number of high performance data processing functions. The list includes mobile phones, basestations, engine management systems and hard-disk drives.
If you have a data processing application which requires a processor versatile enough to carry out both simple but varied housekeeping controller functions. But also with the capability of processing a fast stream of data as efficiently as a traditional Harvard architecture based DSP. Then you would be interested in one of the handful of embedded microprocessor cores with DSP extensions making an appearance in the market.
So what does DSP functionality add to an already high performance 32-bit Risc processor? Essentially it adds a high speed multiply-accumulate (MAC) circuit which can process a very high speed data stream such as video or speech in a mobile phone.
According to ARC, its new 32-bit MAC can be reconfigured as either single 24-bit or as dual 16-bit and is intended to address applications such as audio, mobile phones, Dolby stereo or modems.
To work as a dual MAC, two words of 16-bit data are packed into a 32-bit word. Two 16-bit MACs can be issued on every cycle.
Improved MAC performance figured highly in ARM’s micro/DSP plans. “We needed to significantly improve the performance of our multiplier, especially for 16-bit data,”says Guy Larri, ARM’s CPU product manager. “A lot of DSP code represents data as a fraction and lots of algorithms use saturating arithmetic.”
These are catered for in the new architecture. The multiplier can access either half of the ARM’s 32-bit data registers through a new set of instructions.
The new MAC considerably increases the core’s number crunching potential in target applications like automotive and mobile communications. Simulations have shown that implementing a full duplex G.723.1 speech codec uses 25 per cent of a 160MHz ARM9E’s capability.
This is half that of the “non-DSP” ARM9. Likewise a V.34 soft modem uses 28 per cent of the performance.
If it is raw figures you like then a yardstick could be Infineon Technologies’ (formerly Siemens Semiconductor) TriCore 32-bit microprocessor and DSP hybrid. Built on a 0.25?m process Siemens expects to reach 120Mips at 100MHz with this general purpose device.
Another benefit with the micro/DSP hybrid approach is that the whole may not be bigger than the sum of the parts especially in terms of die size and code density. Texas Instruments, which inevitably has approached the hybrid design by adding microprocessor functionality to its existing DSPcores, claims that the C code for its C27x DSP uses ten per cent less programme memory than the ARM7 Thumb and 45 per cent less than the Motorola HC16.
So where is this likely to lead us? A single processor for mobile handsets says STMicroelectronics which has its own micro/DSP hybrid, the ST100 DSP core. “There are not two worlds – the microcontroller and the DSP – anymore,” said Didier Fuin, chief designer of the ST100 which combines the Risc instructions of a microcontroller with a DSP.
However, Fuin warns against assuming that the micro/DSP hybrid will quickly make the role of the microcontroller in a handset design obsolete. Too much time and money has been spent in developing microcontroller software for designers to drop their existing processors in favour of the newer micro/DSPhybrids.
But the logical benefits of such an approach are such that one suspects it is only a matter of time. Which is probably one reason why the City analysts should get excited by ARM’s automotive strategy.
Although they almost certainly wouldn’t recognise a 32-bit microprocessor core with DSP extensions if one hit them between the eyes. Ah well…


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