PulseIn issue

[JohanDenRidder]

I am trying to stabilize the speed of a motor by reading an attached Hall detector and then feeding the right PWM.
It all works except for some strange behavior of the PulseIn command.
I made a setup with a second pic to provide PortA.5 with a fixed pulse (21 mS, 24Hz).
A logic analyzer on PortA.0 shows the content of the HSerprint commands.

#chip 12F1840,16
#define Sensor PORTA.5
dir Sensor In
dim SensorSpeed as Byte

'USART settings
#define USART_BAUD_RATE 2400
#define USART_TX_BLOCKING
SCKP = 1 ' Invert Tx Bit

Do
      'wait for rising edge
      Do while Sensor=off
      Loop

      PulseIn Sensor, SensorSpeed, ms
      HSerPrint SensorSpeed
 Loop

The LA shows that at the end of the positive pulse on channel 1 there is the serial print showing "21" on channel 0

[JohanDenRidder]

After inserting a wait command the recorded pulse is much shorter:

#chip 12F1840,16
#define Sensor PORTA.5
dir Sensor In
dim SensorSpeed as Byte

'USART settings
#define USART_BAUD_RATE 2400
#define USART_TX_BLOCKING
SCKP = 1 ' Invert Tx Bit

Do
      'wait for rising edge
      Do while Sensor=off
      Loop

      PulseIn Sensor, SensorSpeed, ms
      HSerPrint SensorSpeed

      Wait 75mS
 Loop

Frequency is the same, pulse width is still 21mS,
but GCB measures only 7mS pulse now, see attachment.

[stan cartwright]

"I made a setup with a second pic to provide PortA.5 with a fixed pulse (21 mS, 24Hz)"
24Hz is 41.666666666667 ms
I'm vague about prob. pulsein is frequency independant in gcb..yes? are your pulsein values ok..like ms. just thoughts

[JohanDenRidder]

Stan, please check the attached images. As you can see, the positive pulse is 21mS.
One cycle is 42mS, so just a little over 24Hz.

[mmotte]

Johan,
Your code for waiting for the rising edge doesn't check that the signal is low before rising.

 'wait for rising edge
      Do while Sensor=off
      Loop

So if the sensor is already on , it will go directly to pulseIn and only measure what is left of the pulse.

 'wait for pulse to be low
      Do while Sensor=on
      Loop
 'wait for rising edge
      Do while Sensor=off
      Loop

[JohanDenRidder]

Oops, you are right.
I took this snippet from the help page without thinking for myself...

Thanks,
Johan