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A COMPARISON OF MICROCONTROLLERS AND DSPs

A Case Study
by Jerry Horn

Start ý Fine Tuning ý Turn Up the Volume ý Timing is Everything ý Storage ý Pricing ý Encore ý Sources and PDF

STORAGE

There is one remaining issue that has not been discussed. All of the processors that have been included here cannot store a large amount of data, particularly the microcontrollers. So, how much storage is needed?

I searched for common delay times that are used in commercial guitar multi-effects processors but could not come up with a solid number. It appears that delays are in the 100-ms range, at the most. This appeals to my common sense because I donýt think that longer delays would sound good. Still, Iýll assume an absolute worst-case delay time of 250-ms, just to be conservative.

For 250 ms of storage, a 32-kHz conversion rate, and 16-bit ADC results, 16 KB of storage is needed. As it turns out, it is hard to find small SRAMs. The reason is that large SRAMs are inexpensive. Unfortunately they also require a large number of address and data pins. For 16 KB of memory, the microcontroller must provide a 14-bit address. If the SRAM has a parallel address port and an 8-bit parallel data port, then 22 I/O pins must be used, or glue logic must be added to the design.

I know that someone somewhere must make serial SRAMs with up to 16 KB of storage, but I could not find such a beast. Even with this, the SX28AC would have to bit-bang the serial interface, and this would require almost 800 instructions just to store one 16-bit number and retrieve two 16-bit numbers. So, a parallel device might be required even if a serial device is available.

None of these issues are of concern for the DSP56364. This device is designed to interface directly to serial audio delta-sigma ADCs through a variety of interface formats (I recommend the IIS format). In addition, it can directly interface to AC ý97 codecs, which have a complex interfacing scheme. It also directly supports SRAMs as well as DRAMs. In fact, the interface to memory devices is clever and extremely flexibleýyet another advantage of DSPs. (I am really not trying to sell DSPs here, but these are the facts for this application.)

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