20 MIPS PIC18F8680

Trampas · Guest

I just built up a PCB for a new revision of PCB for a project using the 18F8680. Got the board all running except my serial interface was not working. Scoped the lines and they were triggering, but it was like the BAUD rate was wrong. It hit me like a ton of bricks, I had put a 20Mhz crystal on the board with the 4x PLL!

The part appear to operate perfectly, no heat problems or anything else.

Trampas

dyeatman · Joined: 06 Sep 2003 Posts: 1933 Location: Norman, OK

Trampas,
I had this problem earlier with the 18F452 running at 36.864 with the 4xPLL. My baud rates were off as well because the PLL had not kicked in. Microchip told me that the PLL will not kick in after a code load until you power the chip off and back on. That solved my problem.

To ensure the is code running at the right speed I added a 1 sec heartbeat LED that I check to make sure it blinks at the right rate.

dave

bdavis · Posted: Sat Sep 04, 2004 9:27 am

What do you mean by a "codeload"? IE: bootloader loading new firmware, or maybe ICD downloading new firmware? What if the configuration fuses are already set for 4x PLL - do you still have to do a reset to get the PLL to lock?

dyeatman · Joined: 06 Sep 2003 Posts: 1933 Location: Norman, OK

I have used a both 9.216MHZ oscillator and 18.432 in 4xPLL mode with the ICDU-40. Yes, even with the PLL fuse already set from the previous load sometimes it would not kick in and I have to cycle power but it worked... Amazing bit of overclocking! As interesting as it is I don't begin to trust it for production though.

BTW, this was info I got directly from Microchip tech support when I couldn't get consistant speeds.

In my test setup now I have a normally closed PB in series with 5V to momentarily remove power after a load when the speed isn't right and it has worked every time so far....