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Will Reeve
Joined: 30 Oct 2003
Posts: 209
Location: Norfolk, England
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| rs232 at baud 2 to 5 |
 Posted: Fri Jun 10, 2011 1:23 am |
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Hi,
Wondering if anyone has any code for a software uart running at a baud rate of 2 to 5. It seems the CCS in-built functions only work at 6 and above! I only need transmit. Or is there a work around before I write my own :-( |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19515
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| Posted: Fri Jun 10, 2011 4:21 am |
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I'm surprised the CCS functions allow rates this low. At 2 baud, you are talking 5 seconds to send a single character.
Best way probably, something like (depending on your master clock):
| Code: |
#include <16F877A.h>
#device *=16
#device adc=16
#FUSES NOWDT, HS, PUT, NOLVP
#use delay(clock=20000000)
#define TXBUFFSIZE (32)
char RSTXbuffer[TXBUFFSIZE]; //Make large enough for your longest message.....
int8 buffer_in=0,buffer_out=0;
//Buffer handling tests and code
int8 btemp;
#define isempty(buff,in,out,size) (in==out)
#define hasdata(buff,in,out,size) (!(in==out))
#define isfull(buff,in,out,size) (((in+1)&(size-1))==out)
#define tobuff(buff,in,out,size,chr) { buff[in]=chr;\
in=((in+1) & (size-1));\
if (in==out) out=((out+1) & (size-1));\
}
#define frombuff(buff,in,out,size) (btemp=buff[out],\
out=(out+1) & (size-1), \
btemp)
#define clrbuff(buff,in,out,size) {in=0;\
out=0;}
#define TICKS_PER_BAUD (124) //Change as required
#define TXPIN (PIN_A0) //Change as required
void bputc(char chr) {
tobuff(RSTXbuffer,buffer_in,buffer_out,TXBUFFSIZE,chr);
}
//Tick routine - remember if the TICKS_PER_BAUD, goes over 255, to change ctr
//to an int16
#INT_TIMER2
void tick(void) {
static int8 ctr;
static int8 working;
static int8 bitno;
static int1 sending=FALSE;
if (sending) {
//Code to actually send byte
//Wait for the counter to get to zero
if (--ctr == 0) {
//Now send the next bit
bitno++;
if (bitno<8) {
output_bit(TXPIN,working & 1);
working/=2;
ctr=TICKS_PER_BAUD;
}
else {
//Stop bit or finished
if (bitno==8) {
output_high(TXPIN);
ctr=TICKS_PER_BAUD;
}
else {
sending=FALSE; //Finished byte
}
}
}
}
else {
if (hasdata(RSTXbuff,buffer_in,buffer_out,TXBUFFSIZE)) {
//Here not s3ending, and a byte is in the output buffer
working=frombuff(RSTXbuffer,buffer_in,buffer_out,TXBUFFSIZE); //Get character
sending=TRUE;
bitno=0;
ctr=TICKS_PER_BAUD;
output_low(TXPIN);
}
}
}
void main(void) {
setup_timer_2(T2_DIV_BY_16,251,10); //806 us overflow, 8.0 ms interrupt
clrbuff(RSTXbuff,buffer_in,buffer_out,TXBUFFSIZE);
enable_interrupts(GLOBAL);
enable_interrupts(INT_TIMER2);
//Gives 124 calls per bit
//Then to send:
printf(bputc,"Test message\n");
do {
} while (TRUE);
}
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No guarantees, but it then allows your main code to be doing other things while sending data at this ultra-slow rate....
Best Wishes |
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Will Reeve
Joined: 30 Oct 2003
Posts: 209
Location: Norfolk, England
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| Posted: Fri Jun 10, 2011 7:31 am |
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Thanks. That looks a great foundation. Yes slow rate, slow transmission process! I wish I could tell you more!
Keep well,
Will |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19515
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| Posted: Fri Jun 10, 2011 8:32 am |
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Very low rates, are common, but not quite this low!.
However depending on the exact setup, given just how long it'll take to send a message, and hence the difficulty of asking for a repeat if something goes wrong, I'd suggest you look at hamming codes. Adding 50% to the transmitted data length, would allow detection of two bit errors, and correction of any single bit error.
Best Wishes |
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