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Can't achieve 115200 baud on PIC18F2320 at 6MHz

 
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object01



Joined: 13 May 2004
Posts: 90
Location: Nashville, TN

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Can't achieve 115200 baud on PIC18F2320 at 6MHz
PostPosted: Tue Sep 28, 2004 8:42 am     Reply with quote
According to the formula in the 2320's datasheet, I should be able to achieve 115200 baud within <1% error if I run the internal oscillator at 6000000. But when I setup all this machinery,

Code:
#include <18F2320.h>
#device *=16
#fuses INTRC_IO,NOWDT,PUT,BROWNOUT,NOLVP,NOPROTECT,NOCPD,MCLR   
#use delay (clock=6000000)

#use rs232 (baud=115200, xmit=PIN_C6, rcv=PIN_C7, errors, stream=PHONE)         // Using the hardware USART


The compiler tells me that the baud rate is out of range.

Is the compiler wrong, or is the formula wrong?

--
Jeff S.
Ttelmah
Guest
Re: Can't achieve 115200 baud on PIC18F2320 at 6MHz
PostPosted: Tue Sep 28, 2004 9:15 am     Reply with quote
object01 wrote:
According to the formula in the 2320's datasheet, I should be able to achieve 115200 baud within <1% error if I run the internal oscillator at 6000000. But when I setup all this machinery,

Code:
#include <18F2320.h>
#device *=16
#fuses INTRC_IO,NOWDT,PUT,BROWNOUT,NOLVP,NOPROTECT,NOCPD,MCLR   
#use delay (clock=6000000)

#use rs232 (baud=115200, xmit=PIN_C6, rcv=PIN_C7, errors, stream=PHONE)         // Using the hardware USART


The compiler tells me that the baud rate is out of range.

Is the compiler wrong, or is the formula wrong?

--
Jeff S.

I'd redo your calculation!...
The possible prescalers on this chip, are /16, or /64. With 115200, from a 6MHz clock, you want a total division by 6000000/115200 = 52.083. Using the smaller prescaler gives a multiplier of 3, and division by 3*16 = 48 (SPBRG value=2), with the minimum error of 8.5% (52.083/48). This is well out of acceptable limits.
In this case, I'd say the compiler was 'right'.
I think you might be misreading the data sheet, and accidentally using the _synchronous_ divisor value of 4, which would give the close value you are calculating (13*4=52, error = 0.015%). This is not an available divisor for asynchronous comms.

Best Wishes
object01



Joined: 13 May 2004
Posts: 90
Location: Nashville, TN

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PostPosted: Tue Sep 28, 2004 10:31 am     Reply with quote
Doh!

Thanks. :)

--
Jeff S.
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