At 09:52 15/06/00 +0100, you wrote:
>We use PICs running on 3.7V lithium cells at 32KHz for long battery life.
>Typically our circuits operate between 12 and 18uA. We have used a number of
>processors 16LC58A, 16LC558, 923, 924 etc. Always LC versions for 32KHz and
>Industrial temperature range.
>
>The 58A and the 558 are products which Microchip are superseding with newer
>parts: 58B, 622A (like a 558 but with comparators thrown in). However our
>experience on Microchip's latest die shrinks is that the power consumption
>at 32KHz has gone through the roof - typically x2 or x3.
>
>Has anyone else come across this problem?
>Has anyone got any ideas?
>
>James Wilson

Running at 32KHz is sometime just not the good solution even for current
saving. Another approach that save cost and power is to run your microcontroller
at maximum frequency using internal RC oscillator. As running the MCU
at 32Khz is surely to count time, you may generate an interrupt, say each 10ms
using an external circuitery running at 32Khz. A good and cheap one is a 4060
that is a counter, it integrate also an oscillator that might run with a 32Khz
crystal. So an interrupt can be geenrated at each edge of one of the counter
output, the rest of the time the MCU will enter in sleep mode saving maximum
of current.
Just make a little calcul for PIC16C622A RC osc. mode, 5V operation:
        Icc @ 4Mhz is 3,3 mA (max)
        Ipd        is  20 uA (max)

If you need, say, 1000 clock cycles to process your interrupt and do what
you have to do, you will need:
        1000 x 4 x 0,25us = 1ms @ 3,3mA
if an interrupt occur each 100ms then you will spend 1ms @ 3,3mA and 99ms in
power down mode @ 20uA, so finally the average will give:
        [1x3,3mA]/100 + [99x20uA]/100 = 33uA + 19,8uA = 52.8 uA

This average must be majored by the needed time for the oscillator to restart,
and then it need to add a "IccStart" value that can be quantified by experiment.
Do not forget to add the current needed to run the 4060 !

This just show a high speed can save power in some case and allow high
communication speed when really needed. Sure the calcul show we are still
far from 18uA, but we take only MAX value, an experiment will surely show
lowest current.

Regards,
        Philippe.

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