A COMPARISON OF MICROCONTROLLERS AND DSPs

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A COMPARISON OF MICROCONTROLLERS AND DSPs

Lessons from the Trenches A Case Study
by Jerry Horn

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

I was recently given an opportunity to write about several of my favorite subjectsımicrocontrollers, digital signal processors (DSPs), and mixed-signal components, such as analog-to-digital converters (ADCs) and digital-to-analog converters (DACs).

This opportunity arose when a gentleman who is trying to make a guitar multi-effects unit was looking for information about how to implement digital audio effects using a microcontroller. He had difficulty finding any applications or algorithms for digital effects, so he turned to Circuit Cellarıs ASK US section.

The more I thought about his question, the more convinced I was that the answer really shows the differences between microcontrollers and DSPs, particularly when it comes to signal processing.

Let me first establish a baseline for this article. The gentleman wants to use either a Microchip PIC16F873 or Scenix SX28AC. I am familiar with both of these microcontrollers. He doesnıt want to use a DSP. However, let me choose one for comparison. Because I am familiar with this DSP, I chose the Motorola DSP56364, a processor commonly used in many audio signal processing applications. Table 1 shows a comparison of the three devices.

Processor	Price (US $)	Maximum MIPs	General- purpose I/O	Program RAM	Data RAM	Package
PIC16F873	5.81⁽¹⁾	5 (20 MHz)	24	4096 ı 14	192 ı 8	28-pin
SX28AC	4.05⁽¹⁾	75 (75 MHz)	20	2048 ı 12	136 ı 8	28-pin
DSP56364	11.00⁽²⁾	100 (100 MHz)	16	512ı1280 ı24	100- or 112-pin	1792ı2560 ı 24
Note: ⁽¹⁾ is the price for a quantity of 100 pieces. ⁽²⁾ is the price for each quantity of one.
Table 1ıHere you can see a variety of possible processors for a guitar multi-effect application.

Here are some of the features of each processor:

ı PIC16F873ı10-bit ADC with five input channels, two PWM, three timers, and USART

ı SX28ACı"Virtual Peripherals" implemented in software (PWM, USART, timers, 8-bit ADC, etc.)

ı DSP56364ıExtended serial audio interface (ESAI), serial host interface (SHI), direct interface to DRAM/SRAM, 24 ı 24 multiplier-accumulator, 56-bit barrel shifter, and PLL-based clocking

As you can see, the microcontrollers are inexpensive, small, and flexible. The DSP is larger, more expensive, and more specialized. Note that the only price I could find for the DSP is $11 in low quantities. It might be less expensive in quantities of 100 (Motorola advertises $4.95 for quantities of 250,000).

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