Page 72 - The Art of Designing Embedded Systems

Real Time Means Right Now! 59
to the data), consider using another task or ISR, one driven via a timer
that interrupts at the rate you consider “real time,” to process the queued
data.
An analogous rule to keeping ISRs short is to keep them simple.
Complex ISRs lead to debugging nightmares, especially when the tools
may be somewhat less than adequate. Debugging ISRs with a simple
BDM-like debugger is going to hurt-bad. Keep the code so trivial there’s
little chance of error.
An old rule of software design is to use one function (in this case the
serial ISR) to do one thing. A real-time analogy is to do things only when
they need to ger done, not at some arbitrary rate.
Reenable interrupts as soon as practical in the ISR. Do the hardware-
critical and non-reentrant things up front, then execute the interrupt enable
instruction. Give other ISRs a fighting chance to do their thing.
Fill all of your unused interrupt vectors with a pointer to a null rou-
tine (Figure 4-4). During debug, ulwwys set a breakpoint on this routine.
Any spurious interrupt, due to hardware problems or misprogrammed pe-
ripherals, will then stop the code cleanly and immediately, giving you a
prayer of finding the problem in minutes instead of weeks.

Hardwarre Issues

Lousy hardware design is just as deadly as crummy software. Mod-
ern high-integration CPUs such as the 68332,80186. and 2180 all include
a wealth of internal peripherals-serial ports, timers, DMA controllers,
etc. Interrupts from these sources pose no hardware design issues, since the
chip vendors take care of this for you. All of these chips, though, do per-
mit the use of external interrupt sources. There’s trouble in them thar ex-
ternal interrupts!

Vect-table:
dl start-up power up vector
dl nul 1-1 s r unused vector
dl nu 11-1 s r unused vector
dl timer-isr main tick timer ISR
dl se ri al-i n-i s r ; serial receive ISR
dl se rial-out-i s r ; serial transmit ISR
dl null-isr unused vector
dl null-isr unused vector
null-isr : ; spurious intr routine
imp null-isr ; set BP here!

FIGURE 4-4 Fill unused vectors with a pointer to null-isr, and set a
breakpoint there while debugging.