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Timer interrupt based serial receive routine

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Timer interrupt based serial receive routine
PostPosted: Thu Oct 31, 2013 2:56 am     Reply with quote

This is based on a post I made a long time ago in the forums, showing a way of having async serial receivable on any PIC pin using a time interrupt.


//Timer interrupt based serial rx - timer values shown at 9600bps on a 20MHz PIC4520
//demonstrates a way of retrieving characters from any pin, without having to sit waiting in the main code
//and buffers these for when they are needed....
//9600bps, is about the fastest 'comfortable' with the approach and clock speeds shown.

#include <18F4520.h>
#use delay(crystal=20MHz)
//Pic configuration here for whatever chip you use.

#define RX_INPUT_LINE PIN_A0 //choose the pin you want

#define BUFF_SIZ 16 //definitions for serial buffer
int8 rx_buffer[BUFF_SIZ];
int8 rx_in=0;
int8 rx_out=0;
#define rx_has_data() (rx_in!=rx_out)
#define increment(x) if (++x>=BUFF_SIZ) x=0
char rx_getc(void) //gets a character from the RX buffer if available - return zero if not
   char temp_chr=0;
   if (rx_has_data())
   return temp_chr;

//Remember like all things _get out of the ISR quickly_ This code does this, and any other
//interrupts in use, must also be written this way, if they are not going to interfere.
#int_timer2 //For whatever timer you are using - easiest for timer2 to give accurate times
void timer_int(void) {
   static int8 state=0;
   static int8 incoming=0;
   static int8 bitmask=1;

   //any other timer code wanted here
   switch (state) {
   case 0:
       if (input(RX_INPUT_LINE)==0) state=1; //start bit
   case 1:
   case 2:
       if (input(RX_INPUT_LINE)==0) state++; //test again
       else state=0; //and error restart
   case 3: //since bit time is four interrupts, for three out of four
   case 4: //loops, simply advance one state and exit
   case 5: //This keeps work in the interrupt small
   case 7:
   case 8:
   case 9:
   case 11:
   case 12:
   case 13:
   case 15:
   case 16:
   case 17:
   case 19:
   case 20:
   case 21:
   case 23:
   case 24:
   case 25:
   case 27:
   case 28:
   case 29:
   case 31:
   case 32:
   case 33:
   case 35: //These last three, are delaying to the middle of the stop
   case 36: //bit
   case 37:
       state++; //all that is done in three out of four interrupts....
   //Now sample at four interrupt intervals from the 'double checked'
   //start bit edge - 8 bits
   case 6: 
   case 10:
   case 14:
   case 18:
   case 22:
   case 26:
   case 30:
   case 34:
       if (input(RX_INPUT_LINE)==1) incoming|=bitmask; //set the received bit
       bitmask*=2; //next bit
   case 38:
       //Here should be in the stop bit - save received byte
       if (rx_in==rx_out)
          increment(rx_out); //throw oldest character if buffer overflows

//Now this is based on using a timer interrupt at 4* the baud rate. So for 9600bps, 38400ips
//main stuff
void main(void)
   int8 val_rx;
   //remember you may need to turn things off on the pin you want to use. analogs?.
   //needs to suit the pin you are using.....
   //start with the clock rate - say 20MHz. Instruction rate is 5Meg. For 9600bps require
   //38400 samples per second, and divider of : (5000000/38400) = 130.2 - nearest integer = 130
   //If this is above 256, use a 'divisor' other than one. Otherwise program as:
   setup_timer_2(T2_DIV_BY_1,129,1); //remember one less than the divider required.
   //For 4800bps (5000000/19200) = 260.4, so T2_DIV_BY_4,64,1 etc...
      if (rx_has_data())
         //you will get here when characters have been received
         val_rx=rx_getc(); //retrieves the received character.
         //Do what you want with it......
   while (TRUE);

Makes something much closer to a hardware receive only UART with buffering.

Best Wishes
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