i2c_slave.c

i2c_slave mode for pic

#include        <htc.h>

__CONFIG(INTIO & WDTDIS & PWRTEN & MCLRDIS & UNPROTECT
  & UNPROTECT & BORDIS & IESODIS & FCMDIS);


// SSPSTAT byte is: SMP (7) CKE (6) D/A (5) P (4) S (3) R/W (2) UA (1) BF (0)

#define STATUS_BUFFER_READ_ADD   0x0D //  D/A = 0; S = 1; R/W = 1; BF = 1;
#define STATUS_BUFFER_READ_BYTE  0x2C //  D/A = 1; S = 1; R/W = 1; BF = 0;
#define STATUS_BUFFER_WRITE_ADD  0x09 //  D/A = 0; S = 1; R/W = 0; BF = 1;
#define STATUS_BUFFER_WRITE_BYTE 0x29 //  D/A = 1; S = 1; R/W = 0; BF = 1;
#define STATUS_BUFFER_NACK       0x28 //  D/A = 0; S = 1; R/W = 1; BF = 0;

volatile char flag, data;

main(void)
{
  char c;
  int i;

  PORTB = 0;
  PORTC = 0;

  PIR1 = 0;

  ANSEL = 0;
  ANSELH = 0;
  ADON = 0;

  // Set RB4 & 6 as inputs
  TRISB = 0x50;
  TRISC = 0;

  for (i=0; i<32000; i++) {} // A short delay

  SSPADD = 0x20;

  SSPMSK = 0xFF;

  // Set SSPEN bit of SSPCON (bit 5)
  // Set lower four bits to I2C Slave mode (0110)
  SSPCON = 0x26;

  SSPSTAT &= ( ~ 0x1 ); // clear BF bit

  PIE1 |= 0x08; // enable SSPIE
  INTCON |= 0xC0; // enable PEIE and GIE

  flag = 0;

  // Enable CKP bit
  SSPCON |= 0x10;


  for (;;) {

    c = PORTC;
    
    if ( flag )
      c |= 0x3;
    else
      c |= 0x1;

    PORTC = c;

  }

}

static void interrupt isr(void) {

  char CurrentStatus, dummy;


  // SSPIF
  if ( PIR1 & 0x08 ) {


    PIR1 &= ( ~ 0x08 ) ;
    flag = 1;

    CurrentStatus = SSPSTAT & 0b00101101;

    switch (CurrentStatus) {
      case STATUS_BUFFER_WRITE_ADD:
         dummy = SSPBUF; // dummy read
         break;

      case STATUS_BUFFER_WRITE_BYTE:
         data = SSPBUF; // read in real data
         break;

      case STATUS_BUFFER_READ_ADD:
         SSPBUF = 0xAB;
         SSPCON |= 0x10; // set CKP bit
         break;

      case STATUS_BUFFER_READ_BYTE:
         SSPBUF = 0x55;
         SSPCON |= 0x10; // set CKP bit
         break;

      case STATUS_BUFFER_NACK:
         break;

      default:
         break;

    }

  }

}

 

It's probably fortunate that you don't get an interrupt, because the first one, with all those horrible returns will stop any further interrupts because the GIE is not cleared by a return! So the first thing to say is GET RID OF those returns in the interrupt service routine! The compiler will add the needed retfie instruction at the end of the ISR (which will re-enable global interrupts).
The second suggestion is to use this form to do so:
if()
{}
else if()
{}
else if()
{}
else
{}
I also notice that you seem to do the same action (data = SSPBUF;) for each option except the last, so why not do the exception test first, and do the else afterwards. Less code, more speed. And you're using an int for a flag??? - a bit would do, and save more code and RAM! 

 

Quote: 
--------------------------------------------------------------------------------

Code: 
--------------------------------------------------------------------------------

int c, flag;main(void){  flag = 0;  for (;;) {    if ( flag )      c = 0x3;    else      c = 0x1;

--------------------------------------------------------------------------------



--------------------------------------------------------------------------------


At this point the compiler is perfectly entitled to eliminate the if and set c to 1. You have said (implicitly) to the compiler "flag will not be changed behind your back" (because the C standard assumes a single thread of execution), and then you test this unchanging value. To tell the compiler "flag will be changed behind your back", you must define it thus:
Code: 


--------------------------------------------------------------------------------

volatile bit flag;

--------------------------------------------------------------------------------


(I've made it a bit, and could have used char for slightly larger code, because the PIC can read at most 1 byte at a time from RAM, so might be interrupted between reading the high and low bytes of any variable larger than a char. Since you're only using one bit of that there wouldn't be any real problem, but why not use the most appropriate type? If you were really using a larger-than-char value, then any access to it outside the ISR should be wrapped with di()/ei() - if not, then either your main code or your interrupt routine (or both) could use corrupt values.)

If you were to use data outside the ISR, it too would need to be volatile; if you don't use it outside the ISR then you might as well make it local to that function. 


 

**********************************************

   Program to write a byte to I2C bus,
   using interrupts.

   INPUTS:
      RB4 - SDA
      RB6 - SCL

   OUTPUTS:
      RC0-1 = LEDs (lights when SSPIF interrupts)

   ********************************************* */

#include        <htc.h>

__CONFIG(INTIO & WDTDIS & PWRTEN & MCLRDIS & UNPROTECT
  & UNPROTECT & BORDIS & IESODIS & FCMDIS);


// SSPSTAT byte is: SMP (7) CKE (6) D/A (5) P (4) S (3) R/W (2) UA (1) BF (0)

#define STATUS_BUFFER_READ_ADD   0x0D //  D/A = 0; S = 1; R/W = 1; BF = 1;
#define STATUS_BUFFER_READ_BYTE  0x2C //  D/A = 1; S = 1; R/W = 1; BF = 0;
#define STATUS_BUFFER_WRITE_ADD  0x09 //  D/A = 0; S = 1; R/W = 0; BF = 1;
#define STATUS_BUFFER_WRITE_BYTE 0x29 //  D/A = 1; S = 1; R/W = 0; BF = 1;
#define STATUS_BUFFER_NACK       0x28 //  D/A = 0; S = 1; R/W = 1; BF = 0;

volatile char flag, data;

main(void)
{
  char c;
  int i;

  PORTB = 0;
  PORTC = 0;

  PIR1 = 0;

  ANSEL = 0;
  ANSELH = 0;
  ADON = 0;

  // Set RB4 & 6 as inputs
  TRISB = 0x50;
  TRISC = 0;

  for (i=0; i<32000; i++) {} // A short delay

  SSPADD = 0x20;

  SSPMSK = 0xFF;

  // Set SSPEN bit of SSPCON (bit 5)
  // Set lower four bits to I2C Slave mode (0110)
  SSPCON = 0x26;

  SSPSTAT &= ( ~ 0x1 ); // clear BF bit

  PIE1 |= 0x08; // enable SSPIE
  INTCON |= 0xC0; // enable PEIE and GIE

  flag = 0;

  // Enable CKP bit
  SSPCON |= 0x10;


  for (;;) {

    c = PORTC;
    
    if ( flag )
      c |= 0x3;
    else
      c |= 0x1;

    PORTC = c;

  }

}

static void interrupt isr(void) {

  char CurrentStatus, dummy;


  // SSPIF
  if ( PIR1 & 0x08 ) {


    PIR1 &= ( ~ 0x08 ) ;
    flag = 1;

    CurrentStatus = SSPSTAT & 0b00101101;

    switch (CurrentStatus) {
      case STATUS_BUFFER_WRITE_ADD:
         dummy = SSPBUF; // dummy read
         break;

      case STATUS_BUFFER_WRITE_BYTE:
         data = SSPBUF; // read in real data
         break;

      case STATUS_BUFFER_READ_ADD:
         SSPBUF = 0xAB;
         SSPCON |= 0x10; // set CKP bit
         break;

      case STATUS_BUFFER_READ_BYTE:
         SSPBUF = 0x55;
         SSPCON |= 0x10; // set CKP bit
         break;

      case STATUS_BUFFER_NACK:
         break;

      default:
         break;

    }

  }

}

It's probably fortunate that you don't get an interrupt, because the first one, with all those horrible returns will stop any further interrupts because the GIE is not cleared by a return! So the first thing to say is GET RID OF those returns in the interrupt service routine! The compiler will add the needed retfie instruction at the end of the ISR (which will re-enable global interrupts).
The second suggestion is to use this form to do so:
if()
{}
else if()
{}
else if()
{}
else
{}
I also notice that you seem to do the same action (data = SSPBUF;) for each option except the last, so why not do the exception test first, and do the else afterwards. Less code, more speed. And you're using an int for a flag??? - a bit would do, and save more code and RAM!