Hi David,
thanks for your reply.
Why do you use a relay? It is not needed. The diodes provide the
switch function for you.
I included the relais to ensure that the batteries are cut off 100% when
not needed. If there were only diodes then some current might flow,
given the supply voltage was less than the batteries' voltage.
The PIC may have seen a voltage glitch which sent it into some bad
mode, like CMOS latchup. Cycling the power off then on would clear the
latchup condition.
Power cycling is not the best option, since I'd like the chip to keep
time. ;-)
Do you think the voltage glitch might have been caused by the relais? It
is a reed relais with a diode included, but I always mix it up: The
relais coil input with the internal diode's cathode is connected to
+Vdd, and the coil input with the internal diode's anode is connected to
GND, so that an eventually induced voltage when turning the relais off
will be shorted to GND, right?
I would remove the relay K1 and replace it with a toggle switch to
allow the owner to turn off the batteries if the clock will be
unplugged for a long time.
The switch would be turned ON to provide the battery backup feature.
That is an alternative, sure. But if the relais does not actually
disturb the whole circuit I'd rather leave it, since most people would
forget to turn on the "time keeping" button when plugging it off, right?
Best regards,
Jens
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