Accurate hourly interupt for PIC

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View


Advice wanted please on a suitable source of interupt for a
micrcontroller that meets the following:

1. Low current consumption (will run for at least 6 months on minimal
battery power)

2. Accurate (to within a few minutes a month)

3. Signal for interupt needed every hour (but could cope with other
intervals, eg evey minute or 10 minutes).

I want to build an unattended microcontroller circuit that will run
from a 6 volt lantern battery or smaller for at least 6 months. Hence,
I'm assuming that a crystal oscillator and clock implemented in
software on the microcontroller won't work for me this time due to
current consumption.  I suspect I will need to sleep the
microcontroller (and hence shut down its oscillator) to minimise
current consumption....

however, if I'm wrong on this and there is an accurate low current
oscillator option that will run a PIC, I'd love to hear about that
too.

Cheers
Glenn
                                Glenn Pure
                       Canberra, Australia
Web page: http://www.evans-pure.net

Re: Accurate hourly interupt for PIC


On Sun, 19 Feb 2006 10:40:54 GMT, Glenn Pure

Quoted text here. Click to load it

Are you fixed on using a pic? The msp430 series from ti are very low power,
and have an optional internal oscillator. It also has several versatile timers
that can
generate interrupts. The TI website has examples of using the internal timers as
a RTC.

The PIC10F204 is a 6 pin (sot-23) 100nA sleep current micro.


Re: Accurate hourly interupt for PIC


Quoted text here. Click to load it

Many PICs (or almost any other micro) can meet all your specs with a
32KHz watch crystal which runs continuously and you use a timer to
interrupt and wake up the CPU at the pre-set time interval.
If you don't need fast processing then use the 32KHz clock to do your
processing too, and you may not even need to switch to sleep mode
depending on your battery size.
Otherwise have a faster 4MHz internal oscillator switch on at the
interrupt, do any processing needed and then shut down again.
This is all PIC-101 stuff and is explained in many app notes etc.

Dave :)


Re: Accurate hourly interupt for PIC



Quoted text here. Click to load it

Any nanowatt PIC should do.

Quoted text here. Click to load it

Use a watch crystal to run it.

Quoted text here. Click to load it

A PIC 12F683 uses something like 18uA typical running at 32kHz on 3V and
only 9uA typical at 2V.  As long as you can live without the BOD
(brownout) and WDT (watchdog) that should be it on the current
requirements without sleeping.  Sleeping and using a watch crystal
connected to T1OSC could save you maybe 50% off that on the power.
Probably not really worth the extra effort, unless you want to be able
to wake up and crank the CPU speed up to 8MHz.

A lantern battery won't be necessary, a couple of AA batteries should
easily last for years even running continuously.


Re: Accurate hourly interupt for PIC


Thanks to posters for advice. I don't need any processing grunt. A
32KHz crystal would be fine for me needs.

BTW, I will need to use a PIC with an A/D converter so am somewhat
limited in my choices.

Cheers
Glenn


Quoted text here. Click to load it
                                Glenn Pure
                       Canberra, Australia
Web page: http://www.evans-pure.net

Re: Accurate hourly interupt for PIC



"Glenn Pure" wrote:

Please don't top post.

Quoted text here. Click to load it

How many I/O pins do you need?  The 12F683 has a max of 6 I/O pins, and
the 16F88 has a max of 16 I/O pins.  Both have multiple channels of ADC.



Re: Accurate hourly interupt for PIC


Quoted text here. Click to load it

A/D converters aren't the only way to measure voltage.

Bye.
   Jasen

Re: Accurate hourly interupt for PIC


Quoted text here. Click to load it

Listing all the "nanoWatt" pics with ADCs on the Microchip website
shows that you have no real shortage of choice.
If pin count is not a problem then the 16F88 would probably be the
first choice as it is readily available.
You probably don't need a nanoWatt device either, do the power
calculations for a clock speed of 32.768KHz on a normal PIC and you
might find that will be OK for your requirements too.
BTW, with a clock speed this low you will need to choose the internal
RC oscillator for the ADC clock source, as the clock needs to be within
a specific range.

Dave :)


Re: Accurate hourly interupt for PIC



Quoted text here. Click to load it

It would seem the 16LF88 rather than the 16F88 is needed? The data
sheet suggests the 16F88 isn't rated below 4.0V. Anyone done the
experiment?

If I'll need the 16LF88, any suggestions where to obtain it?

Quoted text here. Click to load it
                                Glenn Pure
                       Canberra, Australia
Web page: http://www.evans-pure.net

Re: Accurate hourly interupt for PIC


Quoted text here. Click to load it

Only if you want to power it from less than 4V
What's worng with 3 or 4 series AA cells?

Quoted text here. Click to load it

It will most likely work, but you won't be able to count on it.
Probably just some spec margin thing.
I've heard of people using them below that without problem.

Quoted text here. Click to load it

Farnell & RS have plenty of LF PICs in other ranges, but not the LF88 I
don't think.
The 16LF877A would be suitable, if you don't mind the 40pin package.

Dave :)


Re: Accurate hourly interupt for PIC


Quoted text here. Click to load it

a "two dollar" watch will do that
get one capabile of producing a short tone every hour,
and connect the output to an amplifier

Quoted text here. Click to load it

Atmel has an application note on their site (or possibly in the datasheet)
where they loop one of the AT90S2313 counters into the external input for
the other to build a 24-bit counter. with a 32768 Hz crystal and a 2.7V
supply the current draw is below 100uA,

there's a trick using the watchdog timer that'll let you shut the chip down
(where current consumption is about 5uA with a 2.7v  supply the watchdog
timer is temperature compensated, and should run at a predicttable rate.

another option would be a "cmos clock" from a PC
the older ones had an 8-bit parallel interface and were capable of producing
an alarm signal at a programmed time (you'd need to set the alarm for 60
minutes into the future) but that may to tie up lots of your IO pins

I'd expect over 1 year from a few alkaline AA batteries for that part

unfortunately none of those suggestions will run directly off a 6V lantern
battery.



Bye.
   Jasen

Re: Accurate hourly interupt for PIC


Quoted text here. Click to load it

Running a micro off a lantern battery for 6 months isn't a huge ask, some
micros will run off a watch battery for years. Although they are probably
less powerful you shouldn't have much trouble getting this to run for close
to the shelf life of the battery :-)

Michael



Site Timeline