Re: Power-on reset




dbernat32 wrote:
> Is there a relatively simple circuit which can assert a power-on reset.
> In other words, the line will go high briefly at power-on and then switch low and remain that way until power-off.
>
> Any ideas would be appreciated.
>
> --dbernat32


How does a spare inverter gate, plus an R, a C, and a diode sound?
(View in fixed font or M$ Notepad):

` VCC VCC
` + +
` | |
` .-. |
` R| | - D
` | | ^ 1/6 74HC14
` '-' |
` | | |\ Reset Pulse
` o----o-----|H>O----o
` | |/
` C---
` ---
` |
` |
` ===
` GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Positive going pulse width (one and only one pulse at turn-on) will be
a little less than R*C seconds. The schottky diode is actually
optional if you have a linear supply that discharges slowly at
turn-off.

This is just about the simplest reset pulse you can get. Remember that
this requires a gate with schmitt trigger input hysteresis to get a
clean reset pulse at turn-on. If you've got a spare NAND gate on
board, try using a 4093 schmitt trigger NAND gate instead of a standard
NAND gate IC. But if a reset pulse with slow fall time is OK, you can
just use any inverter gate (or a NAND or NOR set up as an inverter).

If this doesn't do the job, describe your requirements. There are many
power watchdog/monitor ICs that can do the job for you with equal
simplicity (although they cost more). Do you need brownout protection,
too (reset asserted if power supply goes below a certain voltage)? If
so, one of these ICs or a comparator would be a better choice.

Good luck
Chris

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