PIC18f45k22 serial framing error with 64Mhz clock

Question

I'm trying to transmit serial over the pic18f45k22 eusart peripheral. The messages get sent exactly as expected when the clock is running at 16Mhz, but if I set the PLL to on (so that the the oscillator runs at 64Mhz), I get a framing error.

I have changed the SPBRG registers to account for the new clock frequencey, and tried changing the baudrate generator to both 16 and 8 bit modes, but with no joy.

Current code:

OSCCONbits.IRCF = 0b111; //change Fosc to 16Mhz
OSCTUNEbits.PLLEN = 1; //enable PLL to multiply Fosc by 4


/*Set baud rates and related registers*/
/*For BRG16 = 1 and BRGH = 1, Baud rate = Fosc/(4([SPBRG:SPBRGH]+1)) */
SPBRGH1 = 0; //Set Baud rate control regs to 34 to give baudrate of 115.2k
SPBRG1 = 138;
BAUDCON1bits.BRG16 = 1; //16 bit mode (baudrate generator)
TXSTAbits.BRGH = 1; //Set high speed baud rate 

Thanks in advance, Huggzorx

Answer 1

I'm not familiar with that specific chip but in general, this is what I look at when my UART isn't behaving.

1) Can your clock be divided down to the baud rate with little enough error. Assuming that your baud rate formula in the comments is correct, I think you're okay there:

Baud rate = 16 MHz / (4*(34 + 1)) = 114286  (0.8% error)
Baud rate = 64 MHz / (4*(138 + 1)) = 115107 (0.08% error)

2) Make sure your chip is producing the baud rate you think it should be producing. Some PLLs are really picky about how you turn them on. It's also easy to mis-configure a peripheral. I find that an oscilloscope is your best bet to diagnose this type of problem. If you have access to one, scope the PIC's transmit pin and check that your bit width is 8.68us (1/115200).

If it's 4 times that size (34.72us), then your PLL didn't lock. If it's just a bit off, then the formula might be wrong.

It's not much but hopefully it gets you going in the right direction.