Low voltage/low power design help needed

[dazza]

Hi all,

I have been asked to design a PIC based circuit which needs to be powered by at most 3.0V (2 x AA batteries for example) or even better just one 1.5V AA battery.

What's the lowest voltage a PIC will operate at? (which PIC?)

Nominal current must be around 5uA.

The PIC must be able to count in seconds and must be able to drive a low power LED for short periods.

Could anyone give me some guidlines as to low voltage/power PIC design please?

I'd be interested in battery suggestions to (alternatives to AA's).

Thanks very much in advance.

Regards,
Darren

[jefft]

I think the minimum voltage any PIC will run on is 2 V, although at this voltage, you are somewhat limited in terms of what you can do (A/D must be disabled, and your clock speed must be less than 2 Mhz).

Also, among the PICs I am familiar with, there are none with minimum current requirements less than 0.4 mA (12f675)... most of the datasheets specify 2 mA as the minimum. You can find all of this information in the "electrical characteristics" section of the datasheets.

If one battery is really the concern, you might suggest using lithium batteries, which output 3.7 V. We use the 12f675 powered from a lithium coin cell, and it works fine.

[dbotkin]

I have had great success with the 12F675. Sleep current measures .005 uA on mine. That's with inputs open and pulled up with WPU, nothing at all running - no WDT, BOR, etc. It only gets worse from there. I power my stuff (http://www.hamgadgets.com/store/product_info.php?products_id=51) from a CR2032 lithium coin cell, which lasts for months.

One could use a 32 kHz xtal and leave TIMER1 running to interrupt for timekeeping. 5 uA is going to be very tough, except in sleep mode. If you stay asleep except to service timer events and stuff, your average current will probably be under 5 uA. With ADC disabled you can run down as low as 2V (I've tested it even lower, far lower, but it's not guaranteed).

Start with the number of I/O pins required, add in your absolute must-have features, work it from there. Not enough information in your post for a definite decision, but you have the facts you need, it's just a matter of sorting them out.

Dale

[SteveS]

Look at the 'nanowatt' PICs. Microchip has a good number of app notes on low power design.

If you truly mean 5uA it will get interesting. If that is the average current you'll have to have the PIC sleeping most if the time to make it. The 18LF1320 is around 15uA at 2V with 32kHZ internal clock. Of course that's with it basically doing nothing. Any timers, etc will cost you, not to mention external circuits.

You'll have a hard time finding an LED (visible I assume) that demands less than 1mA.

I love PICs but I must mention the TI MSP430 - supposed to extremely low power - it supposed to have a sub-uA RTC mode which may suit your needs.

I ditto the idea of the lithium - 3+V in one cell, good life, good capacity, light weight.

- SteveS

[Dazza]

Hi,

Thanks for your input thus far - much appreciated.

I said 5uA as I was going by the assumption that an AA is 100mA hour (which could be incorrect) and need the circuit to stay alive for at least 6 months (say 9 for some overhead).

9 months x 31 days x 24 hours = around 4500 hours

100mA / 4500 = 22uA

Allowing further overhead I said 5uA. Perhaps other batteries (of similar small size) are much better than 100mA hour???

The PIC will need to count 6 months worth of time (and do nothing else, just count), then, when the 6 months is up it must illuminate (flash) an LED for 2 weeks.

Perhaps i could use some other circuit/component to do the 6 months timing??? (any suggestions welcome!)

Yes I am familiar with the MSP430 (F149), but wanted something cheaper/smaller and simpler.

I'm sure this is an 8-pin PIC job!

Regards,
Darren

[Ttelmah]

[cyril]

Very roughly figured I get .3ma for 6 months from AAs, and 1/10 of that for a PIC running continuously on a 32KHz crystal. should run for 5 years+ Should be quite easy to do what you want.
Also try a high brightness LED, such as Digikey p/n P408, on 200uA, still quite bright.

[dazza]

Hi thanks again.

Some enlightening information above!.

I'm fairly good with CCS C but am unsure as to how to put the PIC asleep and only wake every so often to perform a count (like once a minute for example) - does anyone have any code to do this?

(Thanks for the LED part number)

Regards,
Darren

[SteveS]

Look at the Timer1 info in the data sheets. The 18F1320 data sheet (and probably others) has a good description of using Timer1 as an RTC. Basically you set up Timer1 as an RTC with an external 32kHz crystal, set up an interrupt off Timer1 and go to sleep. Timer1 can wake the PIC up about once a second and there you will track total time and see if 6mos is up. The CPU will only run a bit every second so average power will be low. Check the data sheets to see how low the current is. Keep supply voltage low to get the biggest benefit.

- SteveS

[dazlogan]

Hi all,

Im going to use a PIC12LC518 running at 32kHz but still need assistance with a/ the battery type and b/ crystal selection.

There will be no other ic's, just the PIC running at 32kHz.

The PIC will never go into sleep mode (I can't do this because the timer cannot awaken it), so it will always be running but this is only a few 10's of uA's.

Battery
=====
I think the CR2032 looks like a good bet but they are relatively expensive so I'm wondering whether cheaper AA's plus a voltage doubler would be better?
Can some one name a cheap voltage doubler or boost IC please?

And if the PIC's minimum voltage is 2.5V, that's not a lot of head room for CR2032 to drop before the minimum PIC voltage is reached.

Best price i've found for CR2032 is a £3.99 for a pack of 5 (Samsung).

Crystal
=====
Does anyone know of a cheap 32kHz crystal - and what capacitors do I need to use with it (please supply a link)

Thanks in advance.

Regards,
Darren

[Neutone]

I have found the battery holder to be more difficult to choose than the battery. They usually cost more so thats a better place to start than the battery. You should plan to run from the full battey voltage and use a lithium. The voltage discharge curve is mostly flat so no regulation is needed. I recoment a 3 or 3.3 volt lithium cell. The coin cells are great but a 2/3A size might be just as cheap and run a great deal longer. I would guess that any 32Khz crystle will be ok. That frequency is only used for low power application.