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C2000 32-bit Microcontrollers Forum
Piccolo F28069 Sys Clock Rate not 80 MHz, odd ePWM rate
I've been using my F28069 control stick and using the EPWM1 to drive the ADC sampling rate. I was assuming the chip was running at 80 MHz, but my sampling rate showed otherwise. I see that a 6 MHz resonator is being used as the input to the pll (verified with scope). I see the software comments and defines (in ...examples.h) indicate a 10 MHz input was expected - 10 MHz * 16 (PLLCR) / 2 (div) = 80 MHz. So I'm guessing this explains part of the sampling rate difference.
I wanted to run the DSC at a rate at least closer to 80 MHz, so I changed the PLLCR value to 12 and the divisor to 1 (defined in the ...examples.h file). I thought this should give me a rate of 6 MHz * 12 / 1 = 72 MHz. However, when when I set the TBPRD to 5000 (CMPR to 1/2 of that, set to count up mode I believe), I was expecting a sample rate of 72 MHz / 5000 = 14.4 kHz, but instead I get a sample rate of 12 KHz.
So, I don't know if the actual system clock rate is 60 MHz instead of 72 MHz (as I programmed), or if there is a division of the system clock to the EPWM clock that I don't know about.
I've checked the forums as best I can, but I don't see anyone saying they think their DSC runs slower than 80 MHz. Am I missing something?
By the way, I thought perhaps the info on PLLCR and divisor might be incorrect, so I changed both to give me the 14.4 kHz sample rate I expected, but the chip began to flake out (ITRAP, etc) so it makes me think I'm running the chip beyond 80 MHz.
Do you have any insight as to why the sample rate is slower than expected? If necessary, I can send code.
You can scale the PWM time base clock with respect to system clock. You may want to check your TBCTL[CLKDIV] and TBCTL[HSPCLKDIV] bit values.
Brad,The 6MHz oscillator is used by the FTDI emulator chip. (not the F2806x device)The F28069 by default works off an internal 10MHz oscillator.Thank you,Brett
After more digging, I think I might not be switching to internal oscillator 1, which does run at 10 MHz...
Thanks Kris, I'll take a look at that. I didn't realize one could scale the PWM time base clock. And thanks for your quick reply, it's a big help!
Yes, you will be using a 10 MHz clock one way or the other. To verify you can configure XCLKOUT on the GPIO mux and scope it out on a pin. You will need to configure it to use a divider so the IO can output the signal accurately.
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