serial port and timer

The have inserted to "if" in such way to make me stamp, alternatively, "0" and "1" to distance of to second. The tails is the following:

#include "c167cs.h"
#include "intrins.h"

int a;
a=0;

void GPT1_viTmr3(void) interrupt 0x23 {

  if(a=0)
     {S0TBUF = '0';
      a=1;}
  else
     {S0TBUF = '1';
      a=0;}

  P2 ^= 0x0001;  /* toggle P2.0 */
}

void main(void) {
  /* setup the serial port */
  S0BG  = 0x0040; /* 9.600 kbaud @ 20 MHz clock */
  S0CON = 0x0011; /* format: 8-N-1 */

  /* P3.10 is used for TxD and P3.11 is used for RxD */
  _bfld_( P3,0x0C00,0x0400);    /**/
  _bfld_(DP3,0x0C00,0x0400);    /* set data direction */

  S0CON |= 0x8000; /* enable baud rate */

  /* use GPT1 for the 1 second time base */
  T3CON = 0x0087;  /* clock tick is 51.2 usec, count down */
  T3    = 0x4C4B;  /* 1 second load value */

  T2CON = 0x0027;  /* t2 reloads T3 on the interrupt */
  T2    = 0x4C4B;

  T3IC  = 0x004B;  /* set up the interrupt level and enable it */
  T3R   = 1;       /* start timer 3 running */

  DP2   = 1;       /* toggle P2.0 every second */

  IEN   = 1;       /* globally enable interrupts */

  for(;;){};
}

The problem is that it stamps only me "1" every second. would someone know me to explain because? Thanks so much.

The have inserted to "if" in such way to make me stamp, alternatively, "0" and "1" to distance of to second. The tails is the following:

#include "c167cs.h"
#include "intrins.h"

int a;
a=0;

void GPT1_viTmr3(void) interrupt 0x23 {

  if(a=0)
     {S0TBUF = '0';
      a=1;}
  else
     {S0TBUF = '1';
      a=0;}

  P2 ^= 0x0001;  /* toggle P2.0 */
}

void main(void) {
  /* setup the serial port */
  S0BG  = 0x0040; /* 9.600 kbaud @ 20 MHz clock */
  S0CON = 0x0011; /* format: 8-N-1 */

  /* P3.10 is used for TxD and P3.11 is used for RxD */
  _bfld_( P3,0x0C00,0x0400);    /**/
  _bfld_(DP3,0x0C00,0x0400);    /* set data direction */

  S0CON |= 0x8000; /* enable baud rate */

  /* use GPT1 for the 1 second time base */
  T3CON = 0x0087;  /* clock tick is 51.2 usec, count down */
  T3    = 0x4C4B;  /* 1 second load value */

  T2CON = 0x0027;  /* t2 reloads T3 on the interrupt */
  T2    = 0x4C4B;

  T3IC  = 0x004B;  /* set up the interrupt level and enable it */
  T3R   = 1;       /* start timer 3 running */

  DP2   = 1;       /* toggle P2.0 every second */

  IEN   = 1;       /* globally enable interrupts */

  for(;;){};
}

The problem is that it stamps only me "1" every second. would someone know me to explain because? Thanks so much.

if (a = 0) { ... }

is an assignment, i.e. it doesn't matter what value a had before your test. You must use == to compare, i.e.

if (a == 0) { ... }

Or reverse the two parts of the if/else and drop the explicit comparison:

if (a) {
    S0TBUF = '1';
    a = 0;
} else {
    S0TBUF = '0';
    a = 1;
}

You can also remove the if statement totally:

S0TBUF = '0' + (a++ & 1);

This extracts the odd/even bit of a, and continuously increments a.

The problem is that I want to stamp "0" then after a second "1", after another second "0", after another second "1" .... ect.With the instruction:

S0TBUF = '0' + (a++ & 1);

does it increase the value progressively?

"does it increase the value progressively?"

Per's post clearly described what it does - read it again, carefully...

As I already mentioned: "This extracts the odd/even bit of a". This means you will alternate between adding 0 and 1 to your printout, i.e. the printout will be 0101010101010101010101...

By the way - why do you want such a printout? It isn't exactly meaningful. You already have a toggling pin. Why emit trace output whenever you toggle the pin? Doesn't your pin toggling work?

Another thing: Why do you declare a as int? You are programming an 8-bit processor and int is in this case a 16-bit data type. Using a data type larger than the native register size is not recommended unless you need the range. In this case, your code only makes use of the values 0 and 1. You could use a 1-byte variable, or you could even modify your code to use a 1-bit variable.

In effects I don't know so much well to use the microcontrollore and I wanted something that alternatively stamped me to video "0" and "1" every second.

Another thing: Why do you declare a as int? You are programming an 8-bit processor and int is in this case a 16-bit data type. Using a data type larger than the native register size is not recommended unless you need the range.

The C167 is a native 16-bit microcontroller.

PS. I added the toggle to P2.0 since the Phytec board has LEDs connected to P2.

"It isn't exactly meaningful."

No - it's a homework assignment!

I was completely convinced that I saw C51 as architecture when I wrote the comment.

Anyway, a number of embedded processors has limited RAM space and special instructions for bit variables, so all variables should get their type based on a consious decision.

Yes, I thought as much, just wanted to point this out for other users.

I would just make a comment to the generated code for the C166 is usually (from a code standpoint) not as efficient when using a byte variable over a word variable (usually you end up with an extra instruction).

Per as you stated the user must decide what memory allocation is important for their application. I also appreciate your suggestion of an add instruction instead of an if statement.