Count Per Second

[oslinux]

Hi all,

i've got a question for you,

I've a Geiger-Muller Tube, an instrument for measuring radiations, which gives me an output in the form of a pulse of minimum 90uS,

i need to measure the number of pulses per second with the minimal possible error,

cause the pulse is minimum 90uS every second i can have at max
1.000.000uS / 90uS = 11.111 CPS (Counts Per Second)

this means that every 5 seconds i have to store the variable in an I2C memory (To be downloaded in a PC)

i was thinking to something like a loop to check for the pulses and store them in a RAM word-sized variable and use an interrupt that every 5 seconds store the variable into the I2C mem,

like this:

GEIGER VAR PORTD.0
CNT VAR WORD
ADD VAR WORD
CONTROL VAR BYTE

CONTROL = $A0
ADD = 0

CheckLoop:
IF GEIGEG THEN
CNT = CNT + 1
ENDIF
GOTO CheckLoop

IntHand:
I2CWRITE SDA,SCL,CONTROL,ADD,[CNT]
ADD = ADD + 1
CNT = 0
RESUME

Do you think it would suit me? (I don't know a lot about pic timing...)
there would be a better way to count or to check for the pulse? (i'm not interested in the pulse lenght)
(Please ignore the missing TMR or INTCONT)

Thank you,

Luca

[HenrikOlsson]

Hi,
If that's the only thing the PIC will do this may work:

GEIGER VAR PORTD.0
CNT VAR WORD
ADD VAR WORD
CONTROL VAR BYTE

Start:
  CNT = 0
  COUNT GEIGER, 5000, CNT     'Count pulses for 5 seconds, store in CNT.
  Gosub SaveData
Goto Start

SaveData:
  'Save data to EEPROM here
RETURN

/Henrik Olsson.

[skimask]

Hi,
If that's the only thing the PIC will do this may work:

GEIGER VAR PORTD.0
CNT VAR WORD
ADD VAR WORD
CONTROL VAR BYTE

Start:
  CNT = 0
  COUNT GEIGER, 5000, CNT     'Count pulses for 5 seconds, store in CNT.
  Gosub SaveData
Goto Start

SaveData:
  'Save data to EEPROM here
RETURN

/Henrik Olsson.

Assuming you don't get over 65,535 pulses...
What happens if you overflow too early? I've never tried, but does COUNT kick out before 5 seconds if it hits 65,535, does it roll over...
And don't forget, COUNT is entirely dependant upon the PICs oscillator speed and will only count low to high transitions (not that either of those should matter much).

[HenrikOlsson]

Hi,

Assuming you don't get over 65,535 pulses...

Yes, but you don't since each pulse is said to be 90uS or longer... About the oscillator speed, yes, COUNT depends on the accuracy of the oscillator but so does any other method I can think of.

/Henrik Olsson.

[oslinux]

Cause i have pulses not shorter than 90uS, i can have a max of 11.111 pulses per second, in 5 seconds i can have a max of 55.555 pulses, there's a way to count for more than 5 seconds, (In other words, there's a way to use a variable longer than a word, like a double word) ?

with a double word i could count up to 4.294.967.296 which is 386.550 seconds!

thank you,

Luca

[ronsimpson]

I had a similar problem where the count might exceed 65000.
I divided up the COUNT into 4 COUNTS.
It can be argued that there is holes between each COUNT. It does work! Don't think about the holes. time=4 seconds
rons
COUNT GEIGER, 1000, CNT1
COUNT GEIGER, 1000, CNT2
COUNT GEIGER, 1000, CNT3
COUNT GEIGER, 1000, CNT4