UART baudrate using internal Osc.

ChawCS · Guest

Hi there,
I need some help on setting the UART baudrate of PIC using CCS C-Compiler.
I am using the internal osc at 8MHz. When I set it to 38400 bps, I get about 41666 bps, using the #use RS232.
I think it been set at low speed (BRGH = 0).
Is there anyway, the BRGH = 1, from calculation that will be about 38461 bps near to 38400 bps that i need.
Thanks
CS

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

1. Post that PIC that you're using.

2. Post your compiler version.

3. Post the #use rs232() statement.

ChawCS · Guest

1) PIC16F914/7

2) CCS C-compiler PCWH 3.249

3) #use rs232(baud = 38400, xmit = PIN_C6, rcv = PIN_C7, parity = N, bits = 8, DISABLE_INTS)

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

ChawCS · Guest

hi PCM programmer,
My calculation are from the PIC datasheet.

Baud Rate Formula
(Asynchronous)
BRGH = 0
Baud Rate = Fosc/(64(x+1))

BRGH = 1
Baud Rate = Fosc/(16(x+1))

1) Now interface with another uP with internal osc at 8MHz also but the bps need to set to about 41666 to get less error.

2) I may use the PIC to inteface with the PC. Unless I can adjust the PC bps to 41666 bps then no problem, but I understand most of the PC software bps are fix at certain standard like 38400 bps.

Do I make a mistake some where?

CS

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

I think I know what you're doing.

1. You're not using the CCS #use rs232() statement.

2. You are setting up the UART registers with your own code.

3. When you solved the baud rate equation for the value of
SPBRG, you did it wrong. You forgot to subtract 1 from
the result. So you used a value of 13 instead of 12.

4. When you put in the wrong value for SPBRG, it made the
baud rate be off by about 8.33% (1/12), because of this
error in the equation.

Solution:
Don't do it yourself. Use the CCS library code. Do it with a
#use rs232() statement.

ChawCS · Guest

Hi PCM Programmer,
I am using the CCS #use RS232, just that i cannot get the right baud rate to interface with the other uP then only i do the calculation to know that the result is about 41666 bps not 38400 bps as i needed. If i am using the asm code it will much easy for me. I dont want to mix my code some using C and some asm.

Ttelmah · Guest

The compiler will actually complain, if the predicted baud rate, is not within about 3.5% of the required value. So it thinks that the value is right, and it has to be using the BRGH bit, or it'd be complaining. Are you sure that the oscillator is running on frequency (the behaviour you are getting, is exactly what is seen if the chip has been programmed with the osccal value set to something like 0).
A quick look at the code generated for the BRG initialisation, using your compiler version, shows the registers are being correctly set, with the divider at 12 (/13), and BRGH=1, to give 38461Hz.
Your problem is probably that your oscillator is not actually running at 8MHz.

Best Wishes