From: Jason White <whitewaterssoftwareinfo@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Fri, 3 Apr 2020 13:23:24 -0700
Subject: Re: [EE] 74HC123, tie nA and B inputs together?
Thread-Topic: [EE] 74HC123, tie nA and B inputs together?
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Okay, that makes sense.

On Friday, April 3, 2020, Bob Blick <bobblick@outlook.com> wrote:

> Hi Jason,
>
> You can't do it the way you suggest. If you look at the internal diagram
> of the '123, both inputs just go to an AND gate with one input inverted.
> There are ways to make it work, like with an RC network, but they are
> either hacks that violate rise/fall time and setup time requirements of H=
C
> logic, or something proper that involves adding more logic such as an XOR
> gate and flip-flop. There may exist some parts that incorporate what you
> want, but I have always done it with FF+XOR, or in a CPLD.
>
> Cheers, Bob
>
> ________________________________________
> From: piclist-bounces@mit.edu <piclist-bounces@mit.edu> on behalf of
> Jason White
> Sent: Friday, April 3, 2020 6:18 AM
> To: Microcontroller discussion list - Public.
> Subject: [EE] 74HC123, tie nA and B inputs together?
>
> Hi everyone,
>
> I have an application where I need to trigger a reset pulse at powerup
> and each time a signal changes from low to high or high to low [2].
> The 74HC123 [1] is a dual monostable multi-vibrator with two trigger
> pins: one that triggers on high-to-low transitions and one that
> triggers at low-to-high transitions.
>
> I am trying to figure out if I can tie the two trigger pins (nA and B)
> together to achieve sensitivity to both rising and falling edges
> without requiring two multi-vibrator elements. An application note [3]
> suggests that one has to use both multi-vibrator elements to get dual
> edge sensitivity. However, a note [4] on the "'123 Function Table" in
> the datasheet [1] suggests (at least to me) that tying both the inputs
> together would result in a triggering each time a signal changes. Does
> anyone have insight on weather or not doing so would result in the
> behavior I need?
>
> Due to the virus it is going to be a while until I can get my hands on
> a physical 74HC123 and test - that is why I am asking for help from
> PICLIST.
>
> [1] http://www.ti.com/lit/ds/symlink/sn74123.pdf
> [2] I actually have 2 signals called DIRECTION and ERROR. I have to
> trigger a reset pulse at powerup and each time either of these signals
> changes (high to low or low to high). I am space constrained such that
> I only have room for one 74HC123. Being able to tie the two inputs
> together would permit me to use one IC for both signals.
> [3] http://www.ti.com/lit/an/slva720/slva720.pdf
> [4] function able note: "These outputs are based on the assumption
> that the indicated steady-state conditions at the A and B inputs have
> been set up long enough to complete any pulse started before the
> setup."
>
> Regards,
> Jason White
> --
>
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> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
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