Page 79 - The Art of Designing Embedded Systems

66 THE ART OF DESIGNING EMBEDDED SYSTEMS

Some, such as the 280, have an external interrupt daisy chain that
serves as a priority encoder. Look at these lines with a scope. If you see the
daisy chain set to a zero, it’s a sure indication that one device did not see
the end-of-interrupt sequence. On the Z80 and Z 180 processors this is pro-
vided by executing the RET1 instruction. Use a normal return instruction
by mistake and you’ll never get another interrupt.
Intel’s x86 family is often used with an 8259 interrupt controller.
Some of the embedded CPUs in this family have 8259-like controllers
built into the processor. If you forget to issue an EO1 (end of interrupt)
command to the 8259 when the ISR is complete, you’ll get that one inter-
rupt only.
You may need to service the peripherals as well before another in-
terrupt comes along. Depending on the part, you may have to read registers
in the peripheral to clear the interrupt condition. UARTs and timers usually
require this. Some have peculiar requirements for clearing the interrupt
condition, so be sure to dig deeply into the databook.

Finding Missing Interrupts

A device that parses a stream of incoming characters will probably
crash very obviously if the code misses an interrupt or two. One that counts
interrupts from an encoder to measure position may only exhibit small
precision errors, a tough thing to find and troubleshoot.
Having worked on a number of systems using encoders as position
sensors, I’ve developed a few tricks over the years to find these missing
pulses.
You can build a little circuit using a single up/down counter that
counts every interrupt and that decrements the count on each interrupt ac-
knowledge. If the counter always shows a value of zero or one, everything
is fine.
Most engineering labs have counters-test equipment that just accu-
mulates pulse counts. I have a scope that includes a counter. Use two of
these, one on the interrupt pin and another on the interrupt acknowledge
pin. The counts should always be the same.
You can build a counter by instrumenting the ISR to increment a
variable each time it starts. Either show this value on a display, or probe
the variable using your debugger.
If you know the maximum interrupt rate, use a performance analyzer
to measure the maximum time in the ISR. If this exceeds the fastest inter-
rupts, there’s very likely a latent problem waiting to pounce.