Using Counters for Ping?

SRLM

Is it possible to use the Propeller counters to read the results from a PING? It's easy enough to bit bang the start pulse, but waiting and blocking for the response is valuable program time wasted. I'd much rather use a counter to get that duration.

mhm21

I have done something similar using a little logic loop. Basically, setup the counter to auto increment when the input pin is high. Processing the read can then take place when the input pin is low and the PHSx register exceeds some threshold. This code is not complete as I cut it out from a larger program, but it shows the flow.

        mov             ctrb,#0                                 'Setup Counter B to monitor high state time of PW pin on MAXBOTIX ping sensor
        mov             ctrb,cntrstate
        mov             frqb,#1

 PING
                        mov     dummy1,ina              ' Sample pins
                        and     dummy1,PING_PIN         ' Limit to PW pin of MaxBotix ping sensor
                        cmp     dummy1,#0 wz            ' If ina==0, between pulses
                        cmp     min_cmp,phsb wc         ' If phsb>0, phsb contains pulse width of previous pulse  
        if_c_and_z      call    #PING_WRITE

PING_RET                ret

cntrstate               LONG    (%11010 << 26 ) | (%001 << 23) | (0 << 9) | (9) 'Last digit is pin declaration 

Tracy Allen

That should be possible Cody. Here is a 'scope shot of the signal sequence, an 8us trigger pulse (orange), followed by a short setup delay, and then the echo timing pulse. In this case the echo is maximum, 20ms. There is time in between the trigger and the echo for Spin to reconfigure the counter from NCO mode to POSDET mode.

CON
  _clkmode = xtal1 + pll8x                '
  _xinfreq = 5_000_000

  NCOMODE = % 00100 << 26
  POSDETMODE = % 01000 << 26

OBJ
  pst : "Parallax Serial Terminal"

PUB PingCount(pin) | trigger
  frqa := 1
  trigger :=  -(clkfreq/1000000 * 8)  ' for 8us duration
  repeat
    ' send an 8us trigger pulse
    dira[pin]~~
    ctra := NCOMODE | pin
    phsa := trigger
    waitpeq(0, |<pin, 0)   ' until end of trigger pulse


    ' capture the return pulse with counter
    dira[pin]~  
    ctra := POSDETMODE | pin
    phsa := 0
      ' do other stuff here
      ' at least 20ms, but less than 26 seconds (at clkfreq=80MHz)
    pause(1000)   ' just a pause for example
    pst.dec(phsa)
    pst.newline

PRI Pause(ms)
  waitcnt(clkfreq/1000*ms+cnt)

SRLM

Thanks for the code. I'll give it a shot and see if I can make this work in C++.

photomankc

I'll see if I can access my C++ code. That's exactly how I measure the TOF from my sonar sensors.