Electronic – Clarification on what the max freq of PIC24FJ64 is

clock-speedfrequencymipspicpll

I'm confusing myself a bit here. The following is taken from this datasheet:

High-Performance CPU:

Modified Harvard Architecture
Up to 16 MIPS Operation @ 32 MHz

Now I want 16 MIPS, which is its max speed. But that 32Mhz, is that the xtal freq or the internal clock freq?

I currently have an 8Mhz xtal selected, and by using the PLL X4, that should give me the 32Mhz, but I'm not sure if the FOSC/2 happens before or after.

What xtal would I need to get max speed out of this device ?

Edit: I believe my current configuration is correct. That an 8Mhz xtal with PLL x4 enabled will give me the 16MIPs i require and that the divide by 2 for the internal clock cycle happens after the PLL.

Best Answer

The datasheet is correct, as they usually are. The PIC 24F family can run at a maximum instruction cycle rate of 16 MHz, which can be expressed as 16 MIPS (Million Instructions per Second). This processor requires 2x clock over the instruction rate, so the maximum clock is 32 MHz. You can achieve 32 MHz clock with a 8 MHz crystal and the 4x PLL. That will result in 16 MHz instruction clock.

Related Solutions

Electronic – the crystal range for a dsPIC30F5016 to get max MIPS

The clock chain is pretty flexible. There may be several different divider and multiplier settings that cause the same ultimate processor clock frequency. However, each part of the chain has a valid frequency range. You have to make sure these aren't exceeded. Depending on the frequencies you are using, there may be divider and multiplier values you can't use because the frequency range of one or more sections would be exceeded.

If you want to see if a 15 MHz crystal would work, walk thru each of the blocks picking divider and multiplier values as you go along, making sure the frequency ranges are adhered to. If you find a solution such that all frequencies are in the valid range, then you can use it.

One advantage of 7.37 MHz versus exactly 7.5 MHz is that the former is a "baud rate" value which makes UART communication over a standard baud rate easier. This is often worth giving up the 1.7% from the maximum speed the processor could operate at.

Electronic – Can’t run PIC24EP128MC206 at rated speed

You should check out example 7-2 in the pic24E family reference manual (FRM) titled "Code Example for Using PLL with 7.37 MHz Internal FRC":

// Select Internal FRC at POR
_FOSCSEL(FNOSC_FRC & IESO_OFF);
// Enable Clock Switching and Configure Primary Oscillator in XT mode
_FOSC(FCKSM_CSECMD & OSCIOFNC_OFF & POSCMD_NONE);

int main()
{
    // Configure PLL prescaler, PLL postscaler, PLL divisor
    PLLFBD=63;            // M=65
    CLKDIVbits.PLLPOST=0; // N2=2
    CLKDIVbits.PLLPRE=0;  // N1=2

    // Initiate Clock Switch to FRC oscillator with PLL (NOSC=0b001)
    __builtin_write_OSCCONH(0x01);
    __builtin_write_OSCCONL(OSCCON | 0x01);

    // Wait for Clock switch to occur
    while (OSCCONbits.COSC!= 0b001);

    // Wait for PLL to lock
    while (OSCCONbits.LOCK!= 1);
}

It looks like the critical step you're missing is __builtin_write_OSCCONL(OSCCON | 0x01);

Also, looking at the math: 7.37*(76/(2*2)) == 140.03MHz which is slightly outside the allowed range, assuming that 140MHz is actually the maximum range (don't ask me why but for some reason it seems like it may be 120MHz).

If this still doesn't work then perhaps there's just an issue with your power supply. The internal FRC oscillator is unstable under temperature and voltage stress, so perhaps you should check to see if you have too much noise. This would make the FRC wonky as well as the VCO used in the PLL, preventing a lock.

If you look at table 30-18 in the pic24EP128MC206 datasheet, it tells you that over the temperature and voltage range you have about a ±1% for some models and ±2% for others. Figure 31-9 shows the variation with a stable voltage over a temperature range. There doesn't appear to be an analysis of voltage variation at a stable temperature.

If you're trying to get a stable run at a high frequency I would just grab a crystal.

EDIT (from comment): So from the sounds of your other posts about your conditions it sounds like you should look elsewhere for a problem. What's the frequency of the ripple? Did you size the internal Vreg capacitor properly? It sounds like since this is a locking issue and not a setting issue you're having some other problems with the board that aren't related to your code.

EDIT:

Glad this turned out to be the right answer! Good luck debugging the rest of the board!

Best Answer

Related Solutions

Electronic – the crystal range for a dsPIC30F5016 to get max MIPS

Electronic – Can’t run PIC24EP128MC206 at rated speed

Related Topic