18F2550 timing

[sstt1976]

Here's a simplified question about something I just noticed:

on an 18F2550 with a 20MHz xtal programmed HS with no postscaler and a DEFINE OSC 20...

Pause 2000 pauses for exactly 2 seconds

but with the same setup...

For cnt=1 to 2000
Usbservice
Pause 1
Next cnt

pauses just for 258ms

Why? First off, I thought I wasn't supposed to be able to Pause 2000 and maintain my USB connection but I am. Secondly, what's with the timing discrepancy?

[Bruce]

How do you know it pauses just for 258ms?

If you are not sure PAUSE 1 works, then setup a simple loop to toggle a pin, and watch
it with a scope.

[sstt1976]

I know those pauses are what they are because the circuit outputs a MIDI note across USB to a host computer and an app on that host measures the time between events. Both examples should theoretically pause 2000ms between each MIDI note indicator .

I'll try the pin toggle to the scope idea. Seems like Pause 1 will take just .129ms because 2000 times through the loop is taking only 258ms for some reason.

[Bruce]

I think you may be forgetting to add the time it takes for certain functions to execute.

PAUSE 2000 pauses for 2000 * 1mS.

For cnt=1 to 2000 ' this takes time
Usbservice ' this takes time
Pause 1 ' this should take about 1mS
Next cnt ' this takes time

Obviously, the 2nd version is NOT the same. You have to account for the time Usbservice
takes + the time to execute the for next loop.

[sstt1976]

What you said could make sense if the 2nd version actually was taking longer, but it's running 8 times faster.

remember, pause 2000 is measuring out as 2000ms but the loop example is completing in 258ms!

[Bruce]

I'm totally guessing here, but it sounds like you might have declared your cnt variable as
a byte VS a word.

[sstt1976]

You got it. Thanks very much!

Now my question is, in the 1st example, how is my USB staying connected all the while during the 'Pause 2000' statement? I though 'Usbservice' needed to be thrown in there almost constantly?