Propeller C language: Why rc_time max = 249,987?

John Kauffman

When I max out time on a Propeller C language rc_time() I get a return of 249987. Why that number? I don't think it is max size for int from C spec: ...Capable of containing at least the [−32767, +32767] range...

In BS2 PBASIC, an RCTIME return that is outside of bounds is 1 (too short) or 0 (too long), which is very convenient.

Thanks.

JasonDorie

This is the code in the simple libs:

long rc_time(int pin, int state)              // rcTime function definition
{
  long tDecay;                                // Declare tDecay variable
  int ctr = ((8 + ((!state & 1) * 4)) << 26); // POS detector counter setup
  ctr += pin;                                 // Add pin to setup
  long tf = st_timeout;                       // Set up timeout - *** this line sets the maximum ***
  long t = CNT;                               // Mark current time
  if(CTRA == 0)                               // If CTRA unused
  {
    CTRA = ctr;                               // Configure CTRA
    FRQA = 1;                                 // FRQA increments PHSA by 1
    input(pin);                               // Set I/O pin to input
    PHSA = 0;                                 // Clear PHSA
    // Wait for decay or timeout
    while((input(pin) == state) && (CNT - t <= tf));
    CTRA = 0;                                 // Stop the counter module
    tDecay = PHSA/st_iodt;                    // Copy result to tDecay
  }
  return tDecay;
}

/**
 * @brief Clock ticks in a time increment used by pulse_in, pulse_out, and rc_time.
 * Default value is the number of system clock ticks in 1/4 s = CLKFREQ/4.
 */
extern int st_timeout;

It looks like there's an st_timeout value that's part of the simple libs, and that is what's setting the maximum time. Because of the way RCtime works, if you had nothing connected at all, the decay would take (more or less) forever, so it has to be bounded somewhere. How long it takes in practice depends on your specific R/C circuit values, so having a timeout value that can be changed might be useful.

249987 is really close to 250,000 so my guess is that they're returning the answer in microseconds, and that would be 1/4 second.

Also, the Prop typically uses 32 bit ints, so values in the range of +/-2,147,000,000 ish.

John Kauffman

Thanks, Jason. I appreciate your observation that it could be changed in the strange case that I could not keep an RC under that time limit.