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arun
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| A very simple question, how can I increase the baud rate |
 Posted: Fri Sep 17, 2004 9:36 am |
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Hi,
Compiler : CCS C Compiler
IDE : MPLAB 6.2
PROCESSOR : PIC 16F877
APPLICATION : Datalogger
I have a very simple question, I have developed a datalogger that uses a PIC 16F877 working on 4 Mhz crystal, at present communication is done using RS 485 at a baud rate of 9600.
I find this very slow , now I would like to increase the speed to maximum baud rate possible.
Kindly suggest ....
1. What crystal should I use, 16 MHz, 20 Mhz
2. What fuse setting should I set ( 9600, 19200 etc.)
thankx
arun |
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drh
Joined: 12 Jul 2004
Posts: 192
Location: Hemet, California USA
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| Posted: Fri Sep 17, 2004 9:46 am |
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I would use an 18.432Mhz xtal. This frequency divides down to an exact standard baud rate.
In your header file, #USE DELAY(CLOCK = 18432000).
Then use SET_UART_SPEED(rate); , rate being equal to the baud rate you want to use. i.e. SET_UART_SPEED(19200);
_________________
David |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Sep 17, 2004 11:14 am |
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The USART in the 16F877 can run at 19200 baud with a 4 MHz crystal.
Run this test program to demonstrate that it works.
(Tested with PCM vs. 3.188)
| Code: | #include <16F877.H>
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock = 4000000)
#use rs232(baud=19200, xmit=PIN_C6, rcv=PIN_C7, ERRORS)
//==================
void main()
{
char c;
printf("Hello World\n\r");
// Echo back characters typed in from the keyboard.
while(1)
{
c = getc();
putc(c);
}
} |
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bdavis
Joined: 31 May 2004
Posts: 86
Location: Colorado Springs, CO
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| Posted: Fri Sep 17, 2004 10:38 pm |
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If you aren't battery operated, I would go with the higher frequency that is a multiple of the baud rate - your baud rate error will be very low, and you have the extra clocks to do more stuff. There is a reason that certain crystal frequencies are popular. You could also go with a 3.6864 MHz crystal, and still do 115,200 baud, but it could use over 50% processor utilization. (I got a interrupt latency of 40 cycles on the PIC18F452).
IMO...  |
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