Like I said before, you can't run a 20Mhz xtal into the HSPLL! The PIC is probably running on the backup internal oscillator (add some code to check for the oscillator fail bit and see what it reads back).Originally Posted by sstt1976
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Like I said before, you can't run a 20Mhz xtal into the HSPLL! The PIC is probably running on the backup internal oscillator (add some code to check for the oscillator fail bit and see what it reads back).
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18F2550 ... Sure you can.
Just need to setup the right divisors in the config's.
<br>
DT
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There I am thinking about the '4685 again. Can't run 20Mhz into it and expect the PIC to run 80Mhz...
(and I was just about to go back and change it too)
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The datasheet says 20MHz is okay to go into HSPLL. You just use a /5 prescaler and the resulting 4MHz is then used to produce a fixed 96MHz PLL reference clock.
What I can't configure however is the 4x slower operation (I want 'pause 1' to actually pause for 4ms!).
I just wanted to see if i could pull this timing thing off without having to use a different xtal. I started everything with 20MHz HS but now that I'm forced to use HSPLL, which is new to me, I know 20 won't work so I'm done with it. The reason I'm moving to HSPLL is because my circuit only enumerates consistently on PC's but not Mac's, unless I use HSPLL instead of HS.
I'm sure both 12 and 24 will work but is there any preference?...unless there is some other way to get the timing I want and still use the 20MHz xtal!
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Actually, it doesn't seem possible no matter what value I choose. Using HSPLL and also somehow getting the program to run 4 times slower is tougher than I thought. Now I will probably start experimenting with changing my ADC_SAMPLEUS value as well as all my PAUSE and PAUSEUS statements...
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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.
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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.
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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.
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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?
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