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'[PIC]: Reducing shutdown power in RC Oscillator mo'
2000\12\06@024703 by Russell McMahon

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Never used PIC in RC mode heretofore.
I'm making a compact RC oscillator device which can draw moderate power
while operating (under 1mA) but MUST be able to be shut down to near zero
current when battery falls too low to prevent rechargeable battery being
damaged. This is a disability aid (specialist timer with beeper or vibration
designed to be worn on a neck thong or carried in a pocket.)

Device operates without power switch until battery goes flat.
Design should have an absolute minimum of components (presently 3 switches,
piezo driven direct from PIC pin, transistor driving a pager vibrator motor
(main power drain), SIP pack for pulldowns, transistor driver resistor, R &
diode for charging, 70 mAH battery).

I had thought that pulling the PIC into permanent reset mode would be
enough.
Horrified to find that in RC mode the PIC draws about 0.4mA at 3 volts due
to the oscillator resistor.

The obvious trick seems to be to use a CMOS output reset IC and drive both
the oscillator resistor and the reset line from this.

As this

"high" current drain when shutdown in RC mode must be a common problem, are
there other clever solutions known?


regards,



           Russell McMahon

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2000\12\06@074838 by Roman Black

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Russell McMahon wrote:
{Quote hidden}

Hi Russell, my recent test of low speed/low power dual
clock using RC clock is at:
http://www.ezy.net.au/~fastvid/pic2clk.htm

I found the 16F84A can be RUN at 60uA using the RC clock
in lower speed modes. This is probably not low enough for
your needs but may answer some of the questions re the RC
power usage.

You state the PIC in RC mode draws 0.4mA, if you use a 100k
osc resistor that goes down to a few uA.

For your app you could try this. Tie the top of the RC
resistor to a PIC pin, instead of 5v. Then when the PIC
is forced into reset the pin will go low so there will be
no resistor current. Or you could tie the top of the R to
the MCLR pin, so whatever solution you use to pull MCLR
low to force reset will hold the osc low as well. This
would give very low power reset. :o)
-Roman

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2000\12\06@092017 by mike

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On Wed, 6 Dec 2000 23:48:08 +1100, you wrote:

>Russell McMahon wrote:
>>
>> Never used PIC in RC mode heretofore.
>> I'm making a compact RC oscillator device which can draw moderate power
>> while operating (under 1mA) but MUST be able to be shut down to near zero
>> current when battery falls too low to prevent rechargeable battery being
>> damaged. This is a disability aid (specialist timer with beeper or vibration
>> designed to be worn on a neck thong or carried in a pocket.)
>>
>> Device operates without power switch until battery goes flat.
>> Design should have an absolute minimum of components (presently 3 switches,
>> piezo driven direct from PIC pin, transistor driving a pager vibrator motor
>> (main power drain), SIP pack for pulldowns, transistor driver resistor, R &
>> diode for charging, 70 mAH battery).
A minor detail - the output from most piezos is very dependent on
running them close to the resonant frequency - I wouldn't think that
an RC oscillator would be stable enough to ensure good output, but
maybe output level isn't too critical in your app.
>> I had thought that pulling the PIC into permanent reset mode would be
>> enough.
>> Horrified to find that in RC mode the PIC draws about 0.4mA at 3 volts due
>> to the oscillator resistor.

How about _powering_ the PIC from the output of a micropower reset
chip - as long as your peripherals can be arranged not to draw  power
when the PIC pins go low this should give extemely low power below the
threshold voltage.

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2000\12\06@103808 by stouchton

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Some more FYI from design experience:

       16F84 running RC at 2V, clock running 32KHz, C's around 100pf, R near 750K,
current under 15uA and forget trying to measure it in reset, less than 1 uA.

       Highly recommned voltage detector to handle reset.  Screwy things happen in
brownout.

{Original Message removed}

2000\12\06@115923 by Dan Michaels

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Mike Harrison wrote:
.........
>>> Device operates without power switch until battery goes flat.
>>> Design should have an absolute minimum of components (presently 3 switches,
>>> piezo driven direct from PIC pin, transistor driving a pager vibrator motor
>>> (main power drain), SIP pack for pulldowns, transistor driver resistor, R &
>>> diode for charging, 70 mAH battery).
>
>A minor detail - the output from most piezos is very dependent on
>running them close to the resonant frequency - I wouldn't think that
>an RC oscillator would be stable enough to ensure good output, but
>maybe output level isn't too critical in your app.


Hi Mike, Russell,

Actually, I have run piezos over a very wide frequency range
off a squarewave from PIC - from say 100 up to 6000 hz. They
may be tuned to a specific frequency, but do give usable
outputs over many. I use a 100-1000 ohm R in series to keep
the volume low enough so's I can stay in the same room with
the @#$%^&* squeeeeeeling.

My device counts pulses to 50 Mhz on an input line and outputs
pulses to the piezo at a rate logarithmically proportional to
the input. Works great. Following is the table I use:

Freq-in                 Piezo pulsefreq
=======                 ===============
1 hz - 10 Khz           100 hz
10 - 20 Khz             200 hz
20 - 40 Khz             500 hz
40 - 80 Khz             750 hz
80 - 160 Khz            1000 hz
160 - 320 Khz           1500 hz
320 - 700 Khz           3000 hz
700 Khz - 50 Mhz        6000 hz

Note, just for fun, I also modulate the same output signal
at 1 - 12 hz, so I can drive an LED for visual indications
off the same pin as the piezo.

- Dan Michaels
Oricom Technologies
http://www.oricomtech.com
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2000\12\09@075558 by Peter L. Peres

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>other solutions

Use a FET to turn on the resistor (between +Vcc and upper end of resistor)
and a PIC pin to turn it off. The FET will need a 5M6 or so bias resistor
to give it the proper opening when the PIC pin is tristate during reset. 2
parts. If you work at 3V then it may be interesting to locate a FET with a
low enough Vp. Down to 1.2V is possible afaik. The FET bias resistor will
consume Vcc/5M6 (or 10M) A when the FET is turned off. This is probably a
fraction of the self discharge current of the battery.

hope this helps,

Peter

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