/* *	Serial port driver for 16Cxx mid-range PIC devices *	(Interrupt-driven) * *	Copyright (C) 1997 HI-TECH Software. *	Author: Jeremy Bennett <jeremy@htsoft.com> *	 *	Comments: *	 *	The major limiting factor in how fast you can go is *	the need to sample the RxData line at several times the *	desired baud rate when looking for a start bit. 8 times is *	used here - less may be possible. With 8 times sampling, the *	maximum baud rate that can be reliably achieved with a 4MHz *	PIC is 1200 baud. This results in about a 100uS interrupt interval, *	with about 30uS used in the interrupt service routine. *	 *	Of course once the start bit has been recognised, the interrupt *	interval could be dropped back to the baud rate, but since it's *	pretty axiomatic that a PIC will wait for characters much more *	than it will actually receive them, this wouldn't help much, and *	would make for additional complication. *	 *	Another approach would be to use the external interrupt pin for the serial input, *	and start timing from the interrupt. This would have to be carefully done *	to allow simultaneous transmission, though. *	 *	This module compiles to about 125 words on a PIC16C84. *	 */#include	<pic.h>#include	<conio.h>#ifdef	_16C71bit adcconversionflag;	// Used by main so it knows when to do a conversion.#endif/************************************* *	Tunable parameters *//* Transmit and Receive port bits */static volatile bit	TxData @ (unsigned)&PORTA*8+3;		/* bit3 in port A */static volatile bit	RxData @ (unsigned)&PORTA*8+2;		/* bit2 in port A */#define	XTAL		4000000	/* Crystal frequency (Hz). */#define	BRATE		1200	/* Baud rate. */#define RX_OVERSAMPLE	8	/* Amount of oversampling the receiver does. Must be				   a power of two */#define SAMPLE_ADC	200	/* sample the ADC this many interrupts/* *	Don't change anything else ************************************/#define TIMER_VALUE	XTAL / (4 * BRATE * RX_OVERSAMPLE)#define TRANSMIT_NUM_BITS	13	// 1 start bit + 8 data bits + 2 stop bits + safe.#if	(RX_OVERSAMPLE-1)&RX_OVERSAMPLE#error	RX_OVERSAMPLE_value must be a power of 2#endif// Receiver states.enum receiver_state {	RS_HAVE_NOTHING,	RS_WAIT_HALF_A_BIT,	RS_HAVE_STARTBIT,	RS_WAIT_FOR_STOP = RS_HAVE_STARTBIT+8};static unsigned char	sendbuffer;		// Where the character to sent is stored.static unsigned char	receivebuffer;		// Where the character is stored as it is received.static bit 		receivebufferfull;	// 1 = receivebuffer is full.#ifdef	_16C71static unsigned char	adcconvertcount;	// How often ADC is read.#endifstatic unsigned char	send_bitno;static unsigned char	receivestate; 	// Initial state of the receiver (0).static unsigned char	skipoversamples;	// Used to skip receive samples.static unsigned char	rxshift;static bit		tx_next_bit;/** * init_uart *  * Initialises the serial port: *  * 	Sets up the I/O directions for the appropriate PortA pins; * 	Sets up Timer0. *  * */voidinit_uart(void){	receivestate = RS_HAVE_NOTHING;	skipoversamples = 1;		// check each interrupt for start bit#ifdef	_16C71 	adcconvertcount = SAMPLE_ADC;	ADCON1 = 2;	// 16C71 requires Port A reconfiguration - make RA2/3 digital I/0,			// leave RA0 and RA1 as analog.#endif	TRISA = 0x17;	// Set up I/O direction.	TRISB = 0xFE;	/* Set up the timer. */	T0CS = 0;			// Set timer mode for Timer0.	TMR0 = (2-TIMER_VALUE);		// +2 as timer stops for 2 cycles					//   when writing to TMR0	T0IE = 1;			// Enable the Timer0 interrupt.	GIE = 1;}voidputch(char c){	while(send_bitno)		continue;	tx_next_bit = 0;	sendbuffer = c;	send_bitno = TRANSMIT_NUM_BITS*RX_OVERSAMPLE;}chargetch(void){	while(!receivebufferfull)		continue;	receivebufferfull = 0;	return receivebuffer;}bitkbhit(void){	return receivebufferfull;}/** * serial_isr *  * Transmits and receives characters which have been * "putch"ed and "getch"ed. *  * This ISR runs BRATE * RX_OVERSAMPLE times per second. *  * */interrupt voidserial_isr(void){	// Reset Timer0 value	// This is added to TMR0 because there is a delay to get to the isr.	PORTB |= 1;	TMR0 += -TIMER_VALUE + 4;	// +2 as timer stops for 2 cycles when writing to TMR0 +2 for tweak	T0IF = 0;#ifdef	_16C71	/*** ADC ***/	/* This will be called every SAMPLE_ADCth time. */	if(--adcconvertcount == 0) {		adcconversionflag = 1;		adcconvertcount = SAMPLE_ADC;	}#endif	/*** RECEIVE ***/	if( --skipoversamples == 0) {		skipoversamples++;		// check next time		switch(receivestate) {		case RS_HAVE_NOTHING:			/* Check for start bit of a received char. */			if(!RxData){				skipoversamples = RX_OVERSAMPLE/2;				receivestate++;			}			break;		case RS_WAIT_HALF_A_BIT:			if(!RxData) {	// valid start bit				skipoversamples = RX_OVERSAMPLE;				receivestate++;			} else				receivestate = RS_HAVE_NOTHING;			break;					// case RS_HAVE_STARTBIT: and subsequent values		default:			rxshift = (rxshift >> 1) | (RxData << 7);			skipoversamples = RX_OVERSAMPLE;			receivestate++;			break;		case RS_WAIT_FOR_STOP:			receivebuffer = rxshift;			receivebufferfull = 1;			receivestate = RS_HAVE_NOTHING;			break;		}	}		/*** TRANSMIT ***/	/* This will be called every RX_OVERSAMPLEth time	 * (because the RECEIVE needs to over-sample the incoming	 * data). */	if(send_bitno) {	       	if((send_bitno & (RX_OVERSAMPLE-1)) == 0) {			TxData = tx_next_bit;		// Send next bit.			tx_next_bit = sendbuffer & 1;			sendbuffer = (sendbuffer >> 1) | 0x80;		}		send_bitno--;	}	PORTB &= ~1;}