Re: Initializing a Flip-Flop on Power-up

"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> schreef in bericht
news:wvoJf.21712$_S7.6970@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hello Spehro,


While you can use an RC (typically about 0.1s time constant with a
diode to help out the protection diode to Vdd, a series resistor to
limit the current in case of a Vdd-to-ground short, and perhaps a
drain resistor from Vdd to Vss to make sure that Vdd falls reasonably
quickly), it's far from foolproof, more complex and not much cheaper.
Another low-cost solution is to use a zener, a BJT and a few
resistors, with or without the RC, but that consumes more power, and
is even more complex.


My circuit du jour is a BSS123. Drain tugs on input that needs reset
low. Gate has cap to VCC and resistor to GND. Plus discharge path if
some BOR is required as well. If the 4-5c are too much I'll sometimes
use a BJT. There must be a discharge path from VCC to GND if power is
turned off. If there isn't that needs some extra parts.

If there is no discharge path of importance, the 4013 stays in position
for weeks. If there is a load on the power supply, the cap gets discharged
through the internal protection diode.

Spending an extra 4-5 cents doesn't sound like you, what have you been
drinking this afternoon? ;)

--
Thanks, Frank.
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