Clock switching hangs when switching back to original source

Question

I am currently working on a project where the clock needs switching between the Primary Oscillator with PLL (POSCPLL) to the FRC and back again.

I have initialised the Clock as the PRIPLL by using the config bits:

#pragma config FNOSC = PRIPLL
#pragma config POSCMOD = HS
#pragma config FPLLMUL = MUL_20
#pragma config FPLLIDIV = DIV_2
#pragma config FPLLODIV = DIV_1
#pragma config FPBDIV = DIV_1
#pragma config FCKSM = CSECME

which sets the OSCCON to 0x01453360 and gives an 80MHz clock signal. I then use this function to switch to the FRC:

INTDisableInterrupts();
//Execute the System unlock sequence
SYSKEY = 0xAA996655;
SYSKEY = 0x556699AA;

OSCCONbits.NOSC = 000;
OSCConfig(OSC_FRC, 0, 0, OSC_FRC_POST_1);
mOSCSetPBDIV(OSC_PB_DIV_1);
//Initiate the clock switch
OSCCONbits.OSWEN = 1;
//Lock SYSKEY
SYSKEY = 0x33333333;
int k = 0;
//Wait until OSWEN = 0 and switch is complete
while (OSCCONbits.OSWEN == 1){
    k++;
}
//Re-enable interrupts
INTEnableInterrupts();
SYSTEMConfig(GetStandbySystemClock(), SYS_CFG_ALL);

To switch back to the POSC PLL i use the same structure but change the value of NOSC to 011; and update the OSCConfig to

OSCConfig(OSC_POSC_PLL, OSC_PLL_MULT_20, OSC_PLL_POST_1, 0); 

Which matches the sequence in the pragma config bits. However, when i run it on the PIC32MX, the debugger on a PICKIT3 cannot get past the function with these set-up bits, however it will allow for it to be set at half the speed (OSC_PLL_POST_2).

If the debugger is paused however, it can be seen that the OSCCON is the same, but when it is resumed, the software restarts.

If anyone could tell me if i have missed a setup bit to enable to switch back to the initial clock or if there is a better way to do this i would be extremely grateful!

Extra information: PIC32MX775F256L, Custom board, 8MHz Crystal (FOXSDLF/080-20), The crystal is connect to the OSC1 and OSC2 (pin 63 and pin 64), loading capacitors 22pF

Thanks in advance

EDIT I have now also tried setting the new clock source without using the OSCConfig function and using the hard values instead.

OSCCONbits.NOSC = 0b011;
OSCCONbits.PLLODIV = 0b000;
OSCCONbits.PLLMULT = 0b101;
OSCCONbits.PBDIV = 0b00;

When PLLODIV is 0b000 the same thing happens, However, when it is set to 0b001 it works and the system is half speed, as it is divided by two

EDIT 2

I have now tried to switch between clocks without using the #pragma config bits I have removed

#pragma config FPLLMUL = MUL_20
#pragma config FPLLODIV = DIV_1
#pragma config FNOSC = PRIPLL
#pragma config FPBDIV = DIV_1

I use the same functions to set up the clock in the first place, and get the desired outcome no problem. I then switch to the FRC clock, again no problems. Only when i try to switch back to the original clock do i encounter a problem. It is also not helped if i switch to the same clock with different parameters first before switching back to the correct ones via the FRC again. But it will work if i switch to the clock with different parameters first, then switch to the correct one via the FRC on start up.

I now have the code isolated to avoid any other code affecting the clock switching and still no difference.

Answer 1

If you meant to set those bits as binary, you need to change this:

OSCCONbits.NOSC = 000;

to this:

OSCCONbits.NOSC = 0b000;

Also wherever you set NOSC back, it should be set to 0b011 not just 011.

A prefix of "0" actually makes the value octal in the XC32 compiler (as pointed out by @Lundin in comments), which is decimal 9, which == 0b1001, so this is certainly one problem that needs fixing.

Also, in the PIC32 Peripheral Library Guide it says:

"In general when switching to a higher CPU clock frequency the Peripheral Bus divisor should be set to the new lower value before changing the CPU frequency"

So, when going up/down speeds, you should be using the appropriate order of the following calls:

OSCConfig(OSC_FRC, 0, 0, OSC_FRC_POST_1);
mOSCSetPBDIV(OSC_PB_DIV_1);