At 12:58 AM 4/13/01 +1200, Russell McMahon wrote:
>I'm using a non-PIC microprocessor with no reset pin and a totally internal
>power on reset and anti-brownout capability. Allegedly.
>No confirmation or denial will be given to people guessing the processor
>type and I'd rather people didn't guess in public at this stage :-).
>
>The spec sheet states a Vdd voltage level below which the processor should
>be in reset mode and above which it should be operational. No requirements
>are placed on the rise time of Vdd to meet reset conditions.
>
>In practice my mileage may and does vary.

You have my sympathies!

No easy answers from me, I'm afraid.  I've dealt with the problem in some
of my designs in 2 different ways and both require added parts.

1) Use a zener regulated supply, with a 4.3V zener in series with the base
of a PNP transistor.  The combination of the Vbe drop and zener voltage
gives me 5V.  Verbally: anode of zener to GND.  Cathode to base of
transistor.  Emitter of transistor to input supply via current limit
resistor.  4k7 resistor from B-E of transistor.  Collector of transistor to
PIC and rest of circuit.

The idea here is to take a slowly rising supply voltage and turn it into a
faster rising voltage.  A lot of my projects use a Zener regulated power
supply - this design adds only 2 parts to that and make for a much more
reliable power-up.

Another variation takes circuit power from the junction of the current
limit resistor and the transistor emitter (5V unswitched) and uses the
collector to drive MCLR (with a 22K pulldown resistor).  I use this version
whenever I have a MCLR pin available for use.

2) Use a 2 transistor power switch configured as a schmitt trigger.  It is
placed in series with whatever regulator I am using for that particular
project: either a zener or 3 terminal regulator.  It does not allow the
regulator to receive power until the input is well above the minimum
required and does not turn off until just above regulator dropout.  I will
sometimes add a 3rd transistor and load resistor to ensure I consume more
current while turned off than while the circuit is powered and operating.

dwayne



Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 17 years of Engineering Innovation (1984 - 2001)

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