From: Brent Brown <brent@eds.co.nz>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Thu, 4 Jun 2020 14:48:20 -0700
Subject: Re: [EE] Detecting a change of level on four lines with digital
	logic/analog circuitry?
Thread-Topic: [EE] Detecting a change of level on four lines with digital
	logic/analog circuitry?
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Hi Jason,

How about: Sum the lines with 4 x resistors, feed this analog voltage into =
the +ve=20
input of one comparator, and -ve input of another comparator. Remaining inp=
uts to=20
a C, with R to analog voltage above, where RC time constant sets pulse peri=
od.=20
Combine comparator outputs with 2 x diodes and a pull-up or pull-down resis=
tor as=20
required.

Brent.

On 4 Jun 2020 at 16:35, Jason White wrote:

> Hi everyone,
>=20
> I have an application where I need to generate at pulse least 1 microseco=
nd
> long any time a rising or falling edge is detected on four digital lines.
> The four signals can change at any time to any direction. The pulse is us=
ed
> to trigger a timer providing a reset signal to the rest of the system.
> Microcontrollers and programmable logic devices aren't allowed.
>=20
> Four 74HC123s will do it, but are too big.
> Four window comparators monitoring the signal through an RC will do it, b=
ut
> also seems too big.
>=20
> Is four schmitt trigger XOR gates with four RC's the smallest
> implementation?
>=20
> I was thinking some sort of analog implementation might be able to do it
> smaller. But AC coupling the four signals together would allow them to
> cancel out. Perhaps a 4-bit encoder/decoder or multiplexer might do it
> better?


--=20
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