[dcbrown]

how about something like this. It should fire a ~20us drive pulse and then listen on the echo line. The echo line should be asserted shortly after the sonar is fired and remain high until the first echo is detected -or- no echo and the line will drop in 36ms.

void ping()
{

unsigned int count;
char usec;
#define drive_pin rc_dig_in01
#define echo_pin  rc_dig_in02

    //~20uS pulse out
    drive_pin = 1;
    for (usec=20; --usec != 0; )
    {
	Nop();
	Nop();
	Nop();
	Nop();
	Nop();
    }
    drive_pin = 0;

    //echo pulse should start shortly... timeout in ~20ms
    count = 35500;
    while(echo_pin == 0)
    {
	count++;
	if (count == 0)
	{
	    printf( "whoa, where'd the pulse go?\n");
	    return;
	}
     }

     // ok, count the pulse width, count*2 is approx. uSec.
    count = 0;
    while (echo_pin != 1)
    {
	Nop(); Nop(); Nop(); Nop();
	count++;
            if (count == 0) break;
    }
    printf( "count = %d\n", count );
    return;
}