From: Bob Blick <bobblick@outlook.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
Sender: "piclist-bounces@mit.edu" <piclist-bounces@mit.edu>
Date: Fri, 3 Apr 2020 10:34:25 -0700
Subject: Re: [EE] 74HC123, tie nA and B inputs together?
Thread-Topic: [EE] 74HC123, tie nA and B inputs together?
Thread-Index: AQHWCbv021jr9Kz/TEepTngPe9IDFahnpdov
Message-ID:
 <BYAPR02MB405584D16E05DD473C6DC100DCC70@BYAPR02MB4055.namprd02.prod.outlook.com>
References:
 <CAOFvGD4oFYQg2pVa7mF1BjzXiBxwNoV4vk6J7nwp0pZrBi3WVQ@mail.gmail.com>
In-Reply-To:
 <CAOFvGD4oFYQg2pVa7mF1BjzXiBxwNoV4vk6J7nwp0pZrBi3WVQ@mail.gmail.com>
Reply-To: Microcontroller discussion list - Public. <piclist@mit.edu>
Accept-Language: en-US
Content-Language: en-US
X-MS-Exchange-Organization-SenderIdResult: Pass
X-MS-Exchange-Organization-PRD: mit.edu
X-MS-TNEF-Correlator:
acceptlanguage: en-US
received-spf: Pass (protection.outlook.com: domain of outlook.com designates
	40.92.4.20 as permitted sender) receiver=protection.outlook.com;
	client-ip=40.92.4.20; helo=NAM02-CY1-obe.outbound.protection.outlook.com;
dkim-signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=outlook.com;
	s=selector1;
	h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck;
	bh=LN2MXi5p/u2XJPZ8fOTg7hDYSoKyvEmVrGja9LZDpfo=;
	b=bcEChmNpjQd3sP2r0QYyGkauRtWYKtN96AAEBsAz0DFcMrS0EwZNkRryVgimdXwTZP/j4Z/Kwg6LoG4EuwIMsAxHXi7vhJuWF6qkWwddvOXhig6q03PhkEssRlOwlzYK4aSTl/g3ebcs27Fgil9MtlEXhNnOCodclyiAD5b0pYJMenNi0MxIKPrV/Tg4QAeD1PHQiVnG4EZZapokKEHRfq6+KH75cX9ew9K2cxfvv+C35mTY6XiXYY7ZtBa5OTTjcARtSCEKrDnXub8pMjYJbt1Q055iab28QVHOjX7/gOXNCdneql00tbw+rFuBpp+UnBzHCsb2AmmzSypI+3/FsQ==
authentication-results: spf=pass (sender IP is 40.92.4.20)
	smtp.mailfrom=outlook.com; mit.edu; dkim=pass (signature was verified)
	header.d=outlook.com;mit.edu; dmarc=pass action=none
	header.from=outlook.com;compauth=pass reason=100
errors-to: piclist-bounces@mit.edu
list-id: "Microcontroller discussion list - Public." <piclist.mit.edu>
list-post: <mailto:piclist@mit.edu>
x-beenthere: piclist@mit.edu
x-mailman-version: 2.1.6
x-topics: [EE]
x-mime-autoconverted: from quoted-printable to 8bit by PCH.mit.edu id
	033HYWxb028666
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0

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. Ther=
e are ways to make it work, like with an RC network, but they are either ha=
cks that violate rise/fall time and setup time requirements of HC logic, or=
 something proper that involves adding more logic such as an XOR gate and f=
lip-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
--

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.