Page 179 - Embedded Microprocessor Systems Real World Design

The only exception is when you know some interrupts are mutually exclusive.
For example, if you are using a half-duplex serial interface, you know you will not
have receive and transmit interrupts simultaneously. Similarly, if your system uses
either an Ethernet or RS232 interface but never both, you can probably assume
that you don’t have to worry about those interrupts stacking up.

Stuck interrupts
As mentioned earlier, a stuck level-sensitive interrupt can cause the processor to
hang up in the ISR. It is possible to detect a stuck interrupt (say, so you can shut
off the motors when it occurs). If you know that the interrupt rate is slow enough
that the polling loop should execute several times between interrupts, set a flag on
each pass through the polling loop. The ISR checks the flag and if it is not set, the
interrupt must be stuck. If the interrupt can occur more than once for each pass
through the polling loop, add a counter to the ISR and increment the count each
time that the ISR executes when the flag is not set. If the count ever reaches a value
higher than should occur in normal operation, the interrupt is stuck. With either
approach, be sure to take into account other ISRs that may delay execution of the
polling loop.

The Shared Memory or VO Trap
Look at the following numbered lines of pseudocode:

1. Read location xyz.
2. OR the value 01 with the data from xyz.
3. Store the result back at xyz.
Now look at the following interrupt code:

OR the value 2Oh into location xyz.
Return.

If xyz contains, say, 00, and the first code (steps 1 to 3) is executed, followed by
the interrupt, the result will be 21 in xyz. If, however, the interrupt occurs while
the processor is between lines 1 and 2 or 2 and 3, the result will be 01 in xyz. The
operation performed by the interrupt is overwritten by the code. You can see this
if you rewrite the original pseudocode with the ISR code in the middle:
1. Read location xyz. (CPU reads 00)
2. OR the value 01 with the data from xyz. (Data = 01, but not stored in xyz
yet.)
-Interrupt occurs here-
OR the value 20 h into location xyz. (This is ISR code; ISR leaves 20 h in xyz.)

160 Embedded Micropocessor Systm