Strange results from bit-banging at 1200 baud

Projects Programming
1

I wrote a simple function to send a string of 8 bit characters on a digital pin using bit-banging. What I got was strange. If I send a message, its first character is often a duplicate of the second character. Say my message is "AXC!", then after repeatedly sending this message, I get this:

XXC!XXC!AXC!AXC!XXC!XXC!XXC!AXC!AXC!XXC!XXC!XXC!AXC!AXC!AXC!XXC!XXC!AXC!AXC!AXC!XXC!XXC!XXC!AXC!AXC!XXC!XXC!XXC!AXC!AXC!XXC!XXC!XXC!AXC!AXC!AXC!XXC!XXC!AXC!AXC!AXC!XXC!XXC!AXC!AXC!AXC!XXC!XXC!AXC!AXC!AXC!XXC!XXC!XXC!AXC!

So sometimes I get it right. Other times instead I get "XXC!".

[UPDATE]
OK, I replaced the noname TTL USB adapter with an arduino and the results are all correct now. So now my new question is: how can this happen with a TTL USB adapter?

My code is below:
I just pass "AXC!" to this function.

void send_to_SDI(char* buf)
{
  int i=0;
  pinMode(SDI_pin,OUTPUT);
  digitalWrite(SDI_pin,marking_level);
  /*
  // Wake up all sensors
  pinMode(SDI_pin,OUTPUT);
  digitalWrite(SDI_pin,break_level);
  delay(break_ms);
  digitalWrite(SDI_pin,marking_level);
  delay(marking_ms);
  */
  noInterrupts();
  while(buf[i]) // Send each byte
  {
    byte out_bit=0;
    //Start bit
    digitalWrite(SDI_pin,spacing_level);
    delayMicroseconds(bit_length_us);
    
    for (int j=0;j<8;j++) // Send each bit, LSB first.
    {
      out_bit=(buf[i]>>j)&1;
      //Serial.print((int)out_bit);
      if (out_bit)
      {
        digitalWrite(SDI_pin,marking_level);
      }
      else
      {
        digitalWrite(SDI_pin,spacing_level);
      }
      delayMicroseconds(bit_length_us);
    }
    //Stop bit
    digitalWrite(SDI_pin,marking_level);
    delayMicroseconds(bit_length_us);
    //Serial.print(" ");
    delayMicroseconds(delay_between_bytes_us);
    i++;
  }
  // Relinquish the SDI pin
  //digitalWrite(SDI_pin,marking_level);
  //pinMode(SDI_pin,INPUT);
  interrupts();
}

I checked a few times but can't find out what's gone wrong. I'll use a different TTL USB adapter and see if I get any better luck. Any help is appreciated.

2

OK, I replaced the noname TTL USB adapter with an arduino and the results are all correct now. So now my new question is: how can this happen with a TTL USB adapter?

3

What value are delay_between_bytes_us and bit_length_us?

4

Nick,

bit_length_us= 833 us (10^6us/1200)
delay_between_bytes_us=3*bit_length_us.

I think the noname adapter might be using PL2303? chip? Will check when I get home. Since I've switched over to the arduino's FTDI chip I've not seen a problem like this anymore. The purpose of my code was to communicate with SDI-12 protocol sensors:

I've since moved on to test my code with two sensors with SDI-12 interface, so far so good. I was able to get temperature and dielectric constant measurements off the sensors. The nice thing about the SDI-12 is you may connect multiple sensors on the same one I/O wire, unlike serial with one I and one O for each device.