UART test program for 16F873

Function
This program echoes characters received from a terminal via RS232.

Hardware
The testboard K3 is set up with reset circuit, crystal oscillator at 4 MHz and a MAX232 for RS232 communication. The 5 capacitors connected to MAX232 can have a smaller value than shown in the circuit diagram, for example 1 uF each. If you use MAX232A or ST232, they can be 100 nF types. The 4 arrows in the circuit diagram indicate signal direction.


Testboard K3 configured for 873uart.asm

Software in assembler

;************************************************************************
; Processor: PIC16F873 at 4 MHz
; Function:  Characters received via RS232 (19200 baud 8-N-1) are echoed
; Hardware:  Testboard K3
; Filename:  873uart.asm  Version 1.2
; Author:    Lars Petersen, oz1bxm@pobox.com
; Credit:    Tony Nixon's 16F877 setup program at
;            http://www.piclist.com/techref/microchip/16f877/setup.htm
; History:   Ver 1.2 Added GOTO Start at 0x0000 to support bootloader
;            Ver 1.1 Replaced clrf STATUS with bcf STATUS,RP0 in TransWt
;            Ver 1.0 Initial release
;*************************************************************************

        list P = 16F873

        include "P16f873.inc"

        __config  _CP_OFF & _XT_OSC & _WDT_OFF & _PWRTE_ON & _LVP_OFF & _BODEN_ON

        CBLOCK 0x20 ;Declare variable address
          dataL
        ENDC
;
; -----------
; INITIALIZE
; -----------
;
        org 0x0000
        goto Start              ; Jump to Start
        org 0x0004
Start   movlw 0x00
        movwf PORTA             ; Initialize port A
        movlw 0x00
        movwf PORTB             ; Initialize port B
        movlw 0x40
        movwf PORTC             ; Initialize port C

        bsf STATUS,RP0          ; RAM Page 1
        movlw 0x00
        movwf TRISA             ; All pins port A output
        movlw 0x00
        movwf TRISB             ; All pins port B output
        movlw 0x80
        movwf TRISC             ; All pins port C output except RC7/RX
;
; -------------------------
; FUNCTION OF PORT A PINS
; -------------------------
;
        movlw 0x06              ; All pins digital I/O
        movwf ADCON1
;
; ------------------------------------
; SET BAUD RATE TO COMMUNICATE WITH PC
; ------------------------------------
; Boot Baud Rate = 19200, No Parity, 1 Stop Bit
;
        movlw 0x0C              ; 0x0C=19200 baud (0x19=9600 baud)
        movwf SPBRG
        movlw b'00100100'       ; brgh = high (2)
        movwf TXSTA             ; enable Async Transmission, set brgh
        movlw b'10010000'       ; enable Async Reception
        bcf STATUS,RP0          ; RAM Page 0
        movwf RCSTA
;
; ------------------------------------
; PROVIDE A SETTLING TIME FOR START UP
; ------------------------------------
;
        clrf dataL
settle  decfsz dataL,F
        goto settle
;
        movf RCREG,W
        movf RCREG,W
        movf RCREG,W            ; flush receive buffer
;
; ----------
; MAIN LOOP
; ----------
;
Main    call Receive            ; wait for a char
        movwf TXREG             ; echo data back to sender
        call Transwt            ; wait until finished sending
        goto Main
;
; ----------------------------
; RECEIVE CHARACTER FROM RS232
; ----------------------------
; This routine does not return until a character is received.
;
Receive btfss PIR1,RCIF         ; (5) check for received data
        goto Receive
        movf RCREG,W
        return
;
; ------------------------------------
; WAIT UNTIL RS232 IS FINISHED SENDING
; ------------------------------------
;
TransWt bsf STATUS,RP0          ; RAM Page 1
WtHere  btfss TXSTA,TRMT        ; (1) transmission is complete if hi
        goto WtHere
        bcf STATUS,RP0          ; RAM Page 0
        return

        END

Remarks
The MAX232 is used for RS232 communication. A simpler circuit can be set up using DS275, see the homepage of Wichit Sirichote from Thailand. A terminal program (HyperTerminal) is set up for 9600 baud with 8 data bits, no parity, no flowcontrol and 1 stop bit.

The same program written in C is here.

Download
Download 873uart.zip containing the source code in assembler and the hexfile.

Credit
Tony Nixon's UART test program at http://www.piclist.com/techref/microchip/16f877/setup.htm was my source for the above program. Thanks!



This page created November 22th, 2001 by Lars Petersen, oz1bxm@pobox.com
Revised September 14th, 2008.

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