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----- Original Message -----
From: "Russell McMahon" <apptech@PARADISE.NET.NZ>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, July 16, 2004 6:01 AM
Subject: [PIC:] Antibrownout circuit for pin limited processors.


> SUMMARY    Anyone have thoughts on brownout protection for low pin count
> processors where reset line is not available?
>
>
> I note that several of the reduced pin count processors have onboard Reset
> BUT do not have anti-brownout circuitry. I am surprised at this (stupid?)
> omission in a modern processor, especially so when the reset pin will
> usually not be available due to the need to utilise every available pin as
> I/O
>
> Both Attiny11 (6 I/O) and the PIC10Fxxx (4 I/O) suffer from this defect.
> If one does not feel lead to use prayer as the primary means of brownout
> protection then a hardware solution is needed.
>
> One approach is to use the reset pin (lose 1 I/O) and use an external
> brownout control chip/cct. IC versions probably exceed the cost of the
> ATtiny11.
>
> With no reset pin one can drop the whole Vdd when brownout is detected
> externally to trigger POR. Running either IC from the output of an
idle-high
> / active low reset generator IC would work.
>
[brownout scheme deleted]

One simple way to do this is with an adjustable shunt regulator like a
TL431. No power to Rpic unless Vcc > 2.5*(1+R1/R2)

For lower voltages, the LMV431 goes down to 1.24V, so the Vbo will be

  Vbo = 1.24*(1+R1/R2)

This part may be more appropriate, since you can use it to specify any
voltage in the PIC's voltage range.

Regards,
 Bob Monsen

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