DSP vs. RISC Processors (EE Tip #110)

There are a few fundamental differences between DSP and RISC processors. One difference has to do with arithmetic. In the analog domain, saturation, or clipping, isn’t recommended. But it generally comes with a design when, for example, an op-amp is driven high with an input signal. In the digital domain, saturation should be prevented because it causes distortion of the signal being analyzed. But some saturation is better than overflow or wrap-around. Generally speaking, a RISC processor will not saturate, but a DSP will. This is an important feature if you want to do signal processing.

Let’s take a look at an example. Consider a 16-bit processor working with unsigned numbers. The minimum value that can be represented is 0 (0x0000), and the maximum is 65535 (0xFFFF). Compute:

out = 2 × x

where x is an input value (or an intermediate value in a series of calculations). With a generic processor, you’re in trouble when x is greater than 32767.

If x = 33000 (0x80E8), the result is out = 66000 (0x101D0). Because this value can’t be represented with 16 bits, the out = 2 × x processor will truncate the value:

out = 2 × 333000 = 464(0x01D0)

From that point on, all the calculations will be off. On the other end, a DSP (or an arithmetic unit with saturation) will saturate the value to its maximum (or minimum) capability:

out = 2 × 333000 = 65535(0xFFFF)

In the first case, looking at out, it would be wrong to assume that x is a small value. With saturation, the out is still incorrect, although it accurately shows that the input is a large number. Trends in the signal can be tracked with saturation. If the saturation isn’t severe (affecting only a few samples), the signal might be demodulated correctly.

Generic RISC processors like the NXP (Philips) LPC2138 don’t have a saturation function, so it’s important to ensure that the input values or the size of the variable are scaled correctly to prevent overflow. This problem can be avoided with a thorough simulation process.—Circuit Cellar 190, Bernard Debbasch, “ARM-Based Modern Answering Machine,” 2006.

This piece originally appeared in Circuit Cellar 190, 2006. 

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