libftdi bitbanged clocksource

bangclock.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
//#include <ftd2xx.h>
#include <errno.h>
//#include <asm/errno.h>
#include <libftdi1/ftdi.h>
#include <time.h>

#include "pins.h"

#define PIN PIN_TX

int main(int argc, char const *argv[])
{
  if(argc < 2){
    printf("Usage: %s <frequency>\n", argv[0]);
    printf("Bitbangs a clock signal of <frequency> Hz (>1Hz <1GHz) on GND<TXD<VCC\n");
    return EINVAL;
  }

  unsigned long freq = strtoul(argv[1],NULL,0);
  if(freq == 0 || errno==ERANGE){
    printf("Could not parse frequency: %s\n", argv[1]);
    return EINVAL;
  }

  struct timespec sleeptime;
  sleeptime.tv_sec=0;
  sleeptime.tv_nsec = 500000000L / freq;

  unsigned char buf=0;
  struct ftdi_context ftdic;
  ftdi_init(&ftdic);
  if(ftdi_usb_open(&ftdic, 0x0403, 0x6001) < 0) {
    printf("Can't open device\n");
    return ENODEV;
  }
  if(ftdi_set_bitmode(&ftdic, PIN,BITMODE_BITBANG)<0) {
    printf("Can't set bitbang mode\n");
    return EIO;
  }

  for(;;) {
    buf ^= PIN;
    ftdi_write_data(&ftdic, &buf, 1);
    //putchar('.');
    nanosleep(&sleeptime,NULL);
  }


  return 0;
}