Hi,

I was always thinking, that a software UART in C on the 8051 was impossible.

Now I needed a transmitter on the AT89C2051, but already all 3 timers are in use.
So I tried it and must revise my former opinion:

<pre>

/************************************************************************/
/* */
/* Software UART transmitter */
/* */
/* Author: Peter Dannegger */
/* danni@specs.de */
/* */
/************************************************************************/
#include < reg51.h >


#define XTAL 24e6

#define BITWAIT (uchar)(XTAL / 12.0 / 19200 + 0.5)
// Attention !!!
// watch this condition:
// at max interrupt time = 35 cycle: 255 - 35 - 7 > BITWAIT > 35 + 7

#define BITSLICE 2 // 9600 baud
//#define BITSLICE 16 // 1200 baud


sbit STXD = P3^5;


char putchar( char c )
{
unsigned char i, j, t;
t = TH0; // T0 in mode 3 !
STXD = 0; // 0 = start bit
for( i = 10; i; i-- ){
for( j = BITSLICE; j; j-- ){ // wait bit time
while( TH0 - t < BITWAIT ); // need 7 cycle
t += BITWAIT;
}
STXD = c & 1; // send bit
c = (unsigned char)c >> 1 | 0x80; // shift 1 in
}
return c;
}

Peter