Andre Abelian wrote:

> 1. I was working on a project that took me 1,5 month to get it done.
> After going thru lots of troubles later on I started using interrupts
> then all of our problems went a way. My question is how do you guys
> calculate or figure out that interrupt is required ?. 

In bigger processors, I usually have at least a timer interrupt. But in
general, I think this depends mostly on the application and somewhat on the
coding style. I create a rough architecture of the application, based on
the requirements what has to be done how fast, with how much jitter, etc.
With this, it's usually not so difficult to see where interrupts come in.
When you do it the first time, it takes a bit, because you have to compare
polling vs interrupts for every part. With time, you make your experiences
and find out about your preferences, and the choice becomes easier. 

> 2. My second question is how do you calculate how much clock speed
> needed? or should I always use 40 mhz. most of the time I use 20 mhz. 

Same as above: find out what the time-critical path is, and what clock
speed you need to get it done. With an interrupt-based architecture, this
is not as simple as counting instruction cycles; you need to take the
interrupts (which not necessarily are in regular intervals) into account.

One easy way to go about this is to start with a medium-high clock rate
(like your 20 MHz) and see how much wiggle room you have. It it's not
enough, you go higher. If it's a lot, you may go lower.

Gerhard

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