Jose Da Silva wrote:

> On April 24, 2005 10:15 pm, Hopkins wrote:
>> In the 18F series there are High & Low priority interrupts.
>>
>> This may seem a bit silly :-) but when should you use each type of
>> interrupt?

> You would use a low priority on a slow device, for example, a keyboard 
> interrupt. The keyboard can wait another couple milliseconds while you 
> service something which needs immediate attention.

In addition to the emergency type examples Jose cited, I'd add anything
where low jitter is important. Using the keyboard interrupt as an example,
you can safely ignore a few ms jitter on such an interrupt. But you can't
ignore a few ms jitter on an async rx/tx interrupt that implements firmware
communication, or when sampling data for a FFT or some such. 

Whether you actually need the two levels depends a lot on your overall
application design. If your application has a quick main loop, your
interrupts don't do much besides caching byte data and flipping bits and
most of the work is done in the main loop, you probably won't need the two
levels. The lower level is mainly meant for interrupt-driven actions that
may take a while and therefore may have to be interrupted by the "real"
interrupts (which would be the high-priority interrupts). 

You can think of the lower level interrupts as an additional application
level between the main loop (which they can interrupt) and what you put
into interrupts (which would be the high-priority interrupts). Or you can
think of the low-priority interrupts as your normal interrupt level, and
put a select few of those interrupts (the extremely time-critical, maybe
the ones where you had jitter problems so far) into the high-priority
level. 

Whether this brings real advantage depends a lot on the application.
Probably low-power applications (with generally low clock speeds relative
to the requirements) are among those that can make good use of this
feature.

Gerhard
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