Basic bitbanging of a PIC microcontroller

        list p=p12f1840
        include <p12f1840.inc>
        __CONFIG _CONFIG1, _MCLRE_OFF & _WDTE_OFF
        __CONFIG _CONFIG2, _LVP_OFF

#define TXPIN 2
#define RXPIN 4

        cblock 0x20
        wait1
        iter
        input
        got
        tmp
        endc

        org 0

        ;;; Define pin 5 (RA2) as output
        movlb 0     ; select bank 0 (PORTA)
        clrf 0x0C   ; clear PORTA
        movlb 2     ; select bank 2 (LATA)
        clrf 0x0C   ; clear LATA
        movlb 3     ; select bank 3 (ANSELA)
        clrf 0x0C   ; clear ANSELA
        movlb 1     ; select bank 1 (TRISA)
        ;; TRISA (0: output, 1: input)
        bcf 0x0C, TXPIN
        bsf 0x0C, RXPIN

        ;;; program
        movlb 0                 ; select bank 0 (PORTA)

        bsf 0x0C, TXPIN         ; init tx

loop0:
        btfsc 0x0C, RXPIN           ; loop while input is high
        bra $-1
        call get

        movf got, 0
        call send

        goto loop0


get:    ;; get contents of W over 1200 baud
        ;; wait 1200 baud (1.2ms)
        ;; wait instructions 500 khz / 4 c/i / 1200 ms = 104.17

        clrf got                 ; ready output
        movlw 0x08
        movwf iter              ; set up iterator

        ;; wait half a tick
        movlw 0x0E
        call waiti

        movlw 0x1F              ; wait past start bit
        call waiti
        goto loop1
shift:
        lsrf got, 1             ; right shift got
loop1:  
        btfsc 0x0C, RXPIN           ; if current bit = 1
        bsf got, 7              ; set got:0 = 1
        ;; wait a tick (100)
        movlw 0x20
        call waiti
        decfsz iter, 1
        goto shift

        return


send:   ;; send contents of W over 1200 baud
        ;; wait 1200 baud (1.2ms)
        ;; wait instructions 500 khz / 4 c/i / 1200 ms = 104.17
        movwf input              ; store W in input

        movlw 0x08
        movwf iter              ; set up iterator

        movlb 0                 ; select bank 0 (PORTA)
        bcf 0x0C, TXPIN             ; transmit start bit

loop2:
        ;; wait 104 - 9 = 95
        movlw 0x1E
        call waiti

        btfss input, 0          ; if bit = 0
        bcf 0x0C, TXPIN         ; transmit a 0
        btfsc input, 0          ; if bit = 1
        bsf 0x0C, TXPIN         ; transmit a 1
        lsrf input, 1           ; left shift 0x20 by 1

        decfsz iter, 1          ; while bits left to transmit
        goto loop2

        ;; wait 102 - 2 = 100
        movlw 0x20
        call waiti

        bsf 0x0C, TXPIN             ; transmit stop bit

        movlw 0x24              ; wait > 104 instructions
        call waiti

        return


waiti:
        ;; W - 0x1E: wait 94 instructions
        ;; W = 0x1F: wait 97 instructions
        ;; W = 0x20: wait 100 instructions
        ;; wait 95 instructions
        ;; 4 + 1 + x * (1 + 2) - 1 = 4 + 3x
        movwf wait1
        decfsz wait1, 1
        bra $-1
        return

        end

// gcc -Wall -lm -O2 -pipe -std=c99 bitbang_terminal.c -o bitbang_terminal
#define _BSD_SOURCE 1

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>
#include <string.h>

int main(int argc, char *argv[]) {
  struct termios term_opt;

  int fd = open("/dev/ttyUSB0", O_RDWR);

  if(fd < 0) {
    perror("argh");
    return -1;
  }

  tcgetattr(fd, &term_opt);

  cfmakeraw(&term_opt);
  cfsetispeed(&term_opt, B1200);
  cfsetospeed(&term_opt, B1200);
  tcsetattr(fd, TCSAFLUSH, &term_opt);

  while (1) {
    char line[256];
    if (NULL == fgets(line, sizeof(line), stdin)) {
      perror("input");
      return -3;
    }
    if (write(fd, line, strlen(line)) < 0) {
      perror("send error");
      return -4;
    }

    for (int i = 0; i < sizeof(line); i++) {
      line[i] = 0;
    }
    sleep(1);
    int read_bytes = read(fd, line, sizeof(line));
    if (read_bytes <= 0) {
      perror("read error");
      return -5;
    }
    line[read_bytes] = '\0';
    printf("received (%d): %s", read_bytes, line);
    printf(" (");
    for (int i = 0; i < read_bytes; i++) {
      if (i > 0) {
        printf(" ");
      }
      printf("0x%X", line[i]);
    }
    printf(")\n");
  }
}