Page 92 - The Art of Designing Embedded Systems

Real Time Means Right Now! 79

Learn about your hardware. Pure software types often have no idea
that the CPU is actively working against them. I talked to an engineer
lately who was moaning about how slow his new 386EX-based instrument
runs. He didn’t know that the 386EX starts with 3 1 wait states and so had
never reprogrammed it to a saner value.

A Poor Man‘s Perfomrance Analyzer
Do keep in tune with the embedded tool industry’s wide range of
performance-analyzing devices. But don’t fail to take detailed measure-
ments just because such a tool is not available. An oscilloscope coupled
to a few spare output bits can be a very effective and cheap performance
analyzer.
Whether you’re working on an 8-bit microcontroller or a 32-bit
VME-based system, always dedicate one or two parallel YO bits to de-
bugging. That is, have the hardware designers include a couple of output
bits just for sofrware debugging purposes. The cost is vanishingly small;
the benefits often profound.
Suppose you’d like to know an ISR’s (or any other sort of routine’s)
precise execution time. Near the beginning of the routine set a debug out-
put bit high; just before exiting return the bit to a zero. For example:
ISR-entry:
push all registers
set output bit high
service interrupt
reset output bit
pop registers
return
Put one scope probe on the bit. You’ll see a pattern that might re-
semble that in Figure 4-8. The ISR is executing when the signal is high.
In this example we see two invocations of the ISR. The first time
(note that the time base setting is 2 msec/division), the routine runs for a bit
over 3 msec. Next time (presumably the routine includes conditional
code), it runs for under 1 msec.
We also clearly see a 14-msec period between executions. If these
two samples are indicative of the system’s typical operation, the total CPU
overhead dedicated to this one interrupt is (3 msec+l msec)/l4 msec. or
29%.
Crank up the scope’s time base and you can measure the ISR’s exe-
cution time to any desired level of precision.