Re: Initializing a Flip-Flop on Power-up
- From: Rich Grise <richgrise@xxxxxxxxxxx>
- Date: Fri, 17 Feb 2006 19:22:08 GMT
On Fri, 17 Feb 2006 18:22:20 +0000, Joerg wrote:
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.
I'm throwing my hat in with Frank on this one. This is way overkill
for what the OP wants.
I wouldn't use 10 uF, however; 1 uF is way more than enough - heck,
what's the time constant of 1 nF and 100K?
And an "R" or an "S" input doesn't need a lot of noise immunity -
once it's reset, it's reset! ;-)
Cheers!
Rich
.
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