From: Clemens Valens <c DOT valens AT mindless DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Using the modem control and status registers
Organization: http://www.remarq.com: The World's Usenet/Discussions Start Here
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Date: Tue, 27 Jul 1999 04:51:32 -0800
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

How do you synchronize the transmitter and the receiver?

You want something like a software UART. Here the receiver
monitors the input (usually through an interrupt) and when
it sees a predetermined change in level (the startbit) it
starts a sample timer. After half a bitlength the receiver
samples the input and that will be the value of the
received bit. This is repeated one bitlenght later, etc.,
until all the bits have been received. For instance:

#define word_lenght     12
#define bit_lenght      37
#define half_bit_length (bit_length>>1)

int index;
serial_word bit[word_length];

void startbit_isr()
{
  index = 0;
  start_sample_timer(half_bit_length);
  clear_interrupt();
}

void sample_timer_isr()
{
  bit[index++] = sample_input();
  if (index<word_length) {
    start_sample_timer(bit_length);
  }
  clear_interrupt();
}

int main(void)
{
  install_interrupt_stuff_etc();

  /* Wait for word to arrive. */
  index = 0;
  while (index<wordlength);
  do_something_with_received_bits();

  return 0;
}

For this you need accurate timers on both sides. Also, you
can do it with polling, but I don't recommend it.

Clemens



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