using delay_ms with 40MHz osc

[mle]

Hello,
I'm using a 40MHz crystal with the PIC18f6520. I am using the processor to read from a dallas 1-wire temperature sensor (DS18S20). I seem to have communication problems when I use the 40MHz crystal, but if I put in a 4MHz crystal the program works fine. The 1-wire communication is time sensitive and I use delay_us() to get the right timing.

Does anyone know of a problem with delay_us() at very high processor speeds? Or does anyone see what else could be the problem?

mle

[rnielsen]

If you want to run the PIC at 40MHZ you will need to use a 10MHZ crystal and use the HS/PLL option in the oscillator configuration. Pull up the data sheet and read up on the oscillator section on how to do it.

Ronald

[prwatCCS]

I was all set to use a PIC18F6720 at 40Mhz - some early info from Microchip suggested it was good to 40Mhz, but then I believe it changed and now they only qualify to 25Mhz. So maybe the 18F6520 is the same.

I am now using the 18F6621 which IS ok for use at 40Mhz.

Check the latest data and errata sheets !!

Of course the comments regarding 40Mhz crystals stand - use 10Mhz xtal and HS/PLL mode. I have used a 40Mhz TTL oscillator source with 18F452 successfully, but not yet with the 18F6xxx devices.

regards
_________________
Peter Willis
Development Director
Howard Eaton Lighting Ltd UK

[rnielsen]

My bad! After looking closer, at the data sheet, the PIC CAN be driven up to 40MHZ.

If your application is time sensitive, remember that interrupts are not figured into the delay_us() function. So, if you have interrupts happening the delay will be extended while the interrupt is serviced.

Ronald

[prwatCCS]

Have you checked that you have the correct setup ??


For my PIC18F6621 at 40Mhz I use

#define CLOCK_FREQUENCY 40000000
#define FOSC 40
#use delay(clock=CLOCK_FREQUENCY)

Those zeros can be easily mistyped !!
_________________
Peter Willis
Development Director
Howard Eaton Lighting Ltd UK

[Ttelmah]