Time "?" running 100Khz on a waitcnt

Zap-o · 2009-04-27 16:00

I am hitting my head against the wall trying to calculate the speed of this loop.

I want to run it around 100Khz for an IC. My prop is running at 80Mhz

PUB ShiftIn (Dpin, Cpin, Mode, Bits) : Value | InBit

   dira[noparse][[/noparse] Dpin ]~
   outa[noparse][[/noparse] Cpin ]:=0 
   dira[noparse][[/noparse] Cpin ]~~  
   InBit~ 
   Value:=0
  REPEAT Bits                                                          
          !outa[noparse][[/noparse] Cpin ] 
          !outa[noparse][[/noparse] Cpin ]  
          InBit:= ina[noparse][[/noparse] Dpin ]
          Value := (Value << 1) + InBit 
          [b]waitcnt(1000 + cnt)[/b]

My thinking is that I am running at [noparse][[/noparse] 80Mhz / 1000 = 80khz] = seconds at 12.5uS. Is this right?

So running at 100Khz I thought that I would change to this

[b]waitcnt(800 + cnt)[/b]

Please inform me if I am making a mistake.

Thanks

Brian Fairchild · 2009-04-27 16:27

It's unlikely it'll run at that speed. SPIN has a maximum speed of around 100,000 lines per second.

You could time the critical section with something like this...

CON     ''General Constants for Propeller Setup
  _CLKMODE = XTAL1 + PLL16X
  _XINFREQ = 5_000_000

OBJ     ''Setup Object references that make this demo work

Ser     :       "FullDuplexSerial"

VAR

long    ClockStart
long    ClockStop
long    ClockDelta

long    maxtime
long    interval


PUB Serial_DEMO

    Ser.start(31, 30, 0, 2400)  ' Initialize serial communication to the PC

    repeat
      ClockStart := cnt

***YOUR CODE HERE***

      ClockStop := cnt

      ClockDelta := ClockStop - ClockStart

      ClockDelta *= 125         ' Convert clock ticks to micro seconds
      ClockDelta /= 1000

      ser.tx(1)                 ' Tell me what the value is through the serial port
      ser.dec(ClockDelta)

Post Edited (Brian Fairchild) : 4/27/2009 4:36:24 PM GMT

MagIO2 · 2009-04-27 18:59

Hi Brian,

you forget that both assignments need some time as well.
Before starting the loop you should do a measurement without any code in between and subtract that from the measurements later on.

ClockStart:=cnt
ClockEnd:=cnt
MeasurementRuntime:=ClockEnd-ClockStart

@Zap-o:
Just remove the waitcnt and see how fast it is then.That's closest you can get to 100kHz using SPIN.
You should remove the InBit. It's an additional assignment which only consumes time.

Time "?" running 100Khz on a waitcnt

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