Counting Pulses during sleep mode

jojos · Joined: 30 Apr 2007 Posts: 64

Hello i am working on a project that measures water flow. The method i use to do that is by counting high to low pulses. Because system works with battery i want to save as much energy as i can. This leads to sleep mode. But is it possible to count the pulses during sleep mode?

Pulses are driven to timer0 pin T0CKI.

I use PIC 18F2520.
Thank you.

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

I would rather suggest to use the INT0 (or INT1, INT2) input, you will have more control counting the pulses in software. (you might need some kind of debouncing anyway: like reading the port again 10-20-50 msec later, to make sure, one pluse is one pulse, and not just the switch bounces,
Do you have a reed switch as the sensing device or is it some kind of electronics ? if it is electronics, then the debouncing may not be required.

So the strategy should be, the pulse egde wakes up the PIC, it increments a counter in software (a int32 variable will do, I think), does some housekeeping if needed and goes back sleeping again waiting for the next pulse.

You also have to decide solving the problem of how to retrieve the data (waking up on data request (data polling) or sending a status message to a host at certain time intervals (data pushing)

There is also the problem of storing the counter in a non-volatile way. or else you will lose the counter value if the battery fails, you cannot however update the value each time in eeprom (writing it uses lots of energy, and also would wear out too quickly)

...these were just a few thoughts.

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

Timer1 and Timer3 on many chips have an asynchronous mode. Would these continue counting in power down mode?

Kenny · Joined: 07 Sep 2003 Posts: 173 Location: Australia

Yes. Counting occurs on the rising edge of the input though. I tested it on the 18F2520. If it's a reed switch then contact bounce could be a problem - should be easily fixed with a resistor and capacitor because the input has a Schmitt buffer.

Would need to count over a fixed period to measure water flow rate, so instead might be better to use either timer 1 or timer 3 with an external 32.768kHz crystal to do the timing and use the external interrupt to do the wakeup and variable incrementing suggested by libor.

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

jojos · Joined: 30 Apr 2007 Posts: 64

Hello again sorry for the delayed response. Using the INT0 interrupt to wake the PIC and then go to sleep I think it wouldn't make any sense cause pulses have high frequency lets say sysyem could have 50 pulses/sec. So I think wake the PIC too often it would be equal from staying awake all the time. I don't know if I am correct.

P.S. The sensing device is a Hall effect Sensor like Hi300.

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

Depending on the oscillator options, e. g. PLL usage, you can expect start-up times in the 0.2 to 2 ms range. The wakeup cycle code can be held rather short in most applications. So there's still a considerable power saving potential with 50 Hz pulse rate, I think. Asynchronous counter operation combined with WDT based wakeup is probably more effective. The sensors static power consumption must be expected higher in case of an electronical sensor, however.

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

jojos · Joined: 30 Apr 2007 Posts: 64

Thank you for your response .About hall effect sensor you are right.I was aware of the consumption i mentioned it to my project manager earlier to take care of it.I am sure we will choose your proposal Libor.As for the pulses i will turmn to the interrupt on change solution.I calculated the time and i saw that the claculations i have to make in every interrupt are maximum of 2 msec.So if i get 20 pulses/sec i will wake the processor for maximum lets say 100 msec with all the times (Stability time after wake e.t.c.)
and so i will have 900 msec sleep and 100 msec awake.Am i right?

The water meter will be used for fields (agricultural purposes)

Libor can you explain to me the debounching problem that you mentioned in your first reply.Thanks again

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

jojos · Joined: 30 Apr 2007 Posts: 64

Libor i saw the datasheet of AH182/183 and to be honest i didn't understand the thing about duty cycle.First of all it has 0.1% duty cycle for AH182 and 25% for AH183.Also there is an awake time of 50 microsec for both and Tperiod time of 50 millisec for AH182 and 200 microsec for AH183.
Which of this characteristics refer to the sample rate of the sensor you mentioned before.

Edit:Ok i think i got it The whole period is for 50 millisec and wakes for 50 microsec.Am i right ?For my case if i have 20 pulses/sec will i get correct measuring or not?

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

AH-182: 0.1% duty cycle = 50us awake 49950us sleeping (TPeriod 50ms = 50000us is the whole cycle)

AH-183 25% duty cycle = 50us awake 150us sleeping (TPeriod 200us is the whole cycle)

For 20 pulses / sec signal, that's a 50 ms period time: the same 50 ms sample rate of the AH-182 would be not enough, a 3-10x oversampling rate is a minimum I would go for a reliable detection.

The AH-183 with its period time of 200us can be a good candidate. It would "listen" to the events (change in magnetic field) 250 times during a 50 ms period of your signal, this is even too much (but no problem)
However you should check the datasheet if they do not artificially slow down the detection rate of this type.

Btw: do you need these counters data to be remotely collected using an RF transceiver for example? or they are just data-loggers to be read at the site.

jojos · Joined: 30 Apr 2007 Posts: 64

Its on the site for now. No rf transmission. Ok so I will pick some samples of the AH183 to test them. Thanks again.