Re: [Sdcc-user] Setting up and sending serial messages

[Peter <pl...@ac...>]

On Fri, 17 Jun 2005, J S wrote:

> My basic goal is to send a "test" message to a serial
> LCD display that requires 9600 baud rate and a 8 bit
> data and 1 bit stop transmission with no parity.  I
> wrote this up to initialize the serial communication:
>
> SCON=0x50;
> TMOD=0x20;
> TH1=0xFD;
> TR1=1;
> T1=1;
>
> This combination seems to be ok, as I can send one
> letter to the display by:
>
> SBUF='T';

Two solutions:

No interrupts:

 	// set TI at program start
 	TI=1;

 	// write characters
 	for(i = 32; i < 96; ++i) { // span ascii set
 		while(!TI);         // wait until TI is 1
 		TI=0;               // clear it
 		SBUF=i;             // send a character;
 	}

With interrupts:

 	volatile bit Txinuse = 0; // whether Txptr is busy
 	char *Txptr;		  // pointer to characters to be sent

 	// s must be static for the duration of the transmission!
 	// i.e. while Txinuse is 1, then it can be reused
 	void putsa(unsigned char *s)
 	{
 	  // note: the next line returns immediately on the 1st call, but blocks
 	  // if the previous string did not go out completely
 	  while(Txinuse); // wait until all chars from a previous putsa call go out
 	  Txptr = s;   // Txptr is used by the ISR
 	  if(*Txptr) { //
 	    SBUF = *Txptr++;
 	    Txinuse = 1;
 	  }
 	}

 	// serial interrupt, shared with RI
 	void serial_int (void) interrupt 4
 	{
 	  if (TI) {               // transmit buffer empty ?
 	    TI = 0;               // clear flag
 	    if(Txinuse) {         // if putsa signalled there is data to send
 	      if(*Txptr)          // while more characters
 	        SBUF = *Txptr++;  // send current char and point past it
 	      else
 	        Txinuse = 0;      // else clear Txptr pointer in use
 	    }
 	  }
 	}

 	// usage:
 	void main(void) {

 		SCON=0x50; // init, as you wrote
 		TMOD=0x20;
 		TH1=0xFD;
 		TR1=1;
 		TI=1;     // == SCON | 0x02
 		P0=0xFF;

 		P0.0=0;   // look for a H->L edge on P0.0
 		putsa("Hello, World!\n"); // returns immediately
 		P0.1=0;   // P0.1 is L quickly after P0.0
 		putsa("Goodbye, World\n"); // blocks until above is sent
 		P0.2=0;   // P0.2 takes a while to go on (about
                           // 14char*104usec*10bits ~= 14.6 msec)

 		while(1); // loop forever
 	}

> ?ASlink-Warning-Undefined Global '_putchar' referenced
> by module 'printf_tiny'

Read the fine manual. You have to define putchar. SDCC has no idea what 
is attached to the circuit, you have to write a function that does the 
actual output. You could output to leds, a lcd, or serial. A synchronous 
putchar suitable for serial from above would be:

// in init code, after SCON=... etc
// you HAVE to set TI at the start of the program
TI=1;

// send a character via serial line, suitable for printf() etc
int putchar(int ch)
{
 	while(!TI); // wait until TI is set (it MUST be set in the init
                     // code for the first character to go out!!!
 	TI=0;       // clear
 	SBUF=ch;    // send it out
 	return((unsigned char)ch); // also return it as per standard C 
}

good luck,
Peter