How to run a DSPIC33FJ128GP706 at 40MIPS from 12MHz xtal?

wpd · Guest

Hello,
I am a newbie to the dsPIC and to CCS. I have the CCS DSP Analog board which has a DSPIC33FJ128GP706 device on it and a 12MHz crystal. I would like to run the cpu at 40 MIPS. From what I've learned, I think there are two possible ways to do this:

1) use the #fuses and #use delay directive to configure the device at boot time:

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

The datasheet seems to be clear in this regard. You can't clock the PLL from a 12 MHz crystal. PLL input frequency range is 0.8 to 8 MHz. You can run it from the internal oscillator (FRC_PLLxx modes), but then you don't get a crystal accurate frequency.

wpd · Joined: 21 May 2009 Posts: 2

Now I'm really confused, because section 9.1.3 of the same data sheet goes on to say that "Fin must be chosen to be in the range of 1.6 MHz to 16
MHz", implying that my 12MHz oscillator should be fine.

Regardless, I cannot figure out how to use the PLL to run the CPU at 40MIPs. Can anybody recommend a source for learning how to do this?

Thanks.

--wpd

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

Yes, you are right. I didn't read the datasheet thorougly.

The 8 MHz limitation applies for PLL startup through crystal oscillator only (see datasheet 2.7). You can use a 12 MHz crystal, but then startup must be performed from FRC and the oscillater switched over to crystal afterwards.

For 40 MIPS respectively 80 MHz Fosc, N1 = 3 and M = 20 can be selected. 12/3*20 = 80

Because I don't have a dsPIC33 at hand to check the settings and also thecorrect handling in PCD, I can't give detailed suggestions how to setup the oscillator.

wpd · Joined: 21 May 2009 Posts: 2

Is there some example code that shows how to start up in FRC mode and then switch to the crystal afterwards? Nothing I've tried seems to work.

--wpd