18F2550 timing

[Bruce]

Why do you want PAUSE 1 to pause for 4mS? Can't you just use PAUSE 4?

Note you do not have to use HSPLL. With a 20MHz osc you can set the PLL divisor to
/5, USBDIV /2, and the USB core will run at 48MHz. You can set FOSC = HS and set the
CPU divisor to CPUDIV = OSC1_PLL2 and use DEFINE OSC 20.

[sstt1976]

The PAUSE was just an example. I need every instruction and every aspect of the program to run 4 times slower than normal.

I already was using HS as you suggested and everything was fine, except enumeration would not carry out through a USB hub into a Mac. It worked in every other condition, PC and Mac. When I use HSPLL it enumerates every time in any situation. Unfortunately, my program works best 4 times slower than it does at normal speed, which was very possible using HS in the scenario I outlined above, but not possible so far using HSPLL.

[Bruce]

So what did you have the CPU divisor set to when it worked with HSPLL set on the MAC?

I know zero about the MAC, but it could be that your main-line program code is too slow
to meet the MAC USB tming. I.E. it could be that a USB device needs to check in more
often than on a PC platform to maintain the connection.

More details would really help.

[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.