Forums

CMOS design rules

Started by Mike Turco July 10, 2004
If I want to tie CMOS (74HCXX) pins hi or low, do I need to use resistors?
If I need resistors, can I connect pins together and then tie them hi/lo
through a single resistor?


On Sat, 10 Jul 2004 13:30:38 -0700, "Mike Turco"
<miketurco@yahoo-nospam4me.com> wrote:

>If I want to tie CMOS (74HCXX) pins hi or low, do I need to use resistors? >If I need resistors, can I connect pins together and then tie them hi/lo >through a single resistor? >
Cmos is a high impeadance input that can be tied directly to either sink or source without resistors, but the general design standard is to use a single resistor to vcc as a "rail tie" for all high ties.
oN 07/10/04, Mike Turco said:

> If I want to tie CMOS (74HCXX) pins hi or low, do I need to use > resistors?
It's good practice.
> If I need resistors, can I connect pins together and then > tie them hi/lo through a single resistor?
Yes. The impedance of a single pin looks much like an open circuit. A single 10K resistor for pull-ups will suffice. For pull-downs, you can wire straight to ground. OTOH, if you're designing something that will go to etch, it is often a useful idea to use separate resistors for each pulled pin (high or low), as you can then press those inputs into service as driven pins without any trace cutting, or even removing the resistors. -- Bill Posted with XanaNews Version 1.16.3.1
<uguess@nowhere.net> wrote in message
news:4on0f0hf8kmlb6ljoqof1nuai0iajop32m@4ax.com...
> On Sat, 10 Jul 2004 13:30:38 -0700, "Mike Turco" > <miketurco@yahoo-nospam4me.com> wrote: > > >If I want to tie CMOS (74HCXX) pins hi or low, do I need to use
resistors?
> >If I need resistors, can I connect pins together and then tie them hi/lo > >through a single resistor? > > > Cmos is a high impeadance input that can be tied directly to either > sink or source without resistors, but the general design standard is > to use a single resistor to vcc as a "rail tie" for all high ties.
Why????? I have seen many configuration pins on CPUs (also CMOS) directly tied to ground or VCC. Dip switches on microcontroller inputs (also CMOS) are often directly tied to ground. So what good does a resistor do? Meindert
oN 07/10/04, Meindert Sprang said:

> Why????? > > I have seen many configuration pins on CPUs (also CMOS) directly tied > to ground or VCC. Dip switches on microcontroller inputs (also CMOS) > are often directly tied to ground. So what good does a resistor do?
One reason it used to be recommended back in the old days (30 years ago, when 74C was new) was to fend off latch-up if there were multiple power lines involved. The more practical reason, in my view, is simply to allow for those "oops" situations where easy access to another input term may save you from problems... -- Bill Posted with XanaNews Version 1.16.3.1
"Meindert Sprang" <mhsprang@NOcustomSPAMware.nl> wrote:

>Why?????
>I have seen many configuration pins on CPUs (also CMOS) directly tied to >ground or VCC. Dip switches on microcontroller inputs (also CMOS) are often >directly tied to ground. So what good does a resistor do?
Most likely a hang over from ancient TTL practice. 74 series TTL has (had) an abs max supply voltage rating of 7v but an abs max input voltage rating of 5.5v. Tying inputs directly to VCC reduced maximum supply voltage tolerence by 1.5v. Tying with pull up resistors was supposed to protect the inputs from excessive VCC which would not otherwise kill the chip. Unless the semis you are using have similar characteristics tying up or down with a single resitor is pretty pointless. Tying with multiple resistors lets you force nodes during test and make modding PCBs easier.
"Meindert Sprang" <mhsprang@NOcustomSPAMware.nl> wrote in message
news:10f0qfuqvdhgq25@corp.supernews.com...
> <uguess@nowhere.net> wrote in message > news:4on0f0hf8kmlb6ljoqof1nuai0iajop32m@4ax.com... > > On Sat, 10 Jul 2004 13:30:38 -0700, "Mike Turco" > > <miketurco@yahoo-nospam4me.com> wrote: > > > > >If I want to tie CMOS (74HCXX) pins hi or low, do I need to use > resistors? > > >If I need resistors, can I connect pins together and then tie them
hi/lo
> > >through a single resistor? > > > > > Cmos is a high impeadance input that can be tied directly to either > > sink or source without resistors, but the general design standard is > > to use a single resistor to vcc as a "rail tie" for all high ties. > > Why????? > > I have seen many configuration pins on CPUs (also CMOS) directly tied to > ground or VCC. Dip switches on microcontroller inputs (also CMOS) are
often
> directly tied to ground. So what good does a resistor do? > > Meindert >
1. Current limiting in case of power-supply glitches 2. Isolation from other inputs which if chips failed could become outputs If any outputs are used to drive inductive loads like relay coils, then spikes are a real possibility 3. Provide an easily accessed pad for circuit changes 4. Waste board space 5. Increase parts count and costs 6. Enrich resistor manufacturers. ... Since I mostly design with TTL (and compatible) devices, I tend to use lots of resistors -- unless I'm wire-wrapping my own board in which case I'll use as few connections as possible. Seriously, in an all-CMOS system there are probably very few reasons to use the resisitors for connecting unused inputs to supply rails. Norm
On Sun, 11 Jul 2004 00:11:34 +0200, "Meindert Sprang"
<mhsprang@NOcustomSPAMware.nl> wrote:

><uguess@nowhere.net> wrote in message >news:4on0f0hf8kmlb6ljoqof1nuai0iajop32m@4ax.com... >> On Sat, 10 Jul 2004 13:30:38 -0700, "Mike Turco" >> <miketurco@yahoo-nospam4me.com> wrote: >> >> >If I want to tie CMOS (74HCXX) pins hi or low, do I need to use >resistors? >> >If I need resistors, can I connect pins together and then tie them hi/lo >> >through a single resistor? >> > >> Cmos is a high impeadance input that can be tied directly to either >> sink or source without resistors, but the general design standard is >> to use a single resistor to vcc as a "rail tie" for all high ties. > >Why????? > >I have seen many configuration pins on CPUs (also CMOS) directly tied to >ground or VCC. Dip switches on microcontroller inputs (also CMOS) are often >directly tied to ground. So what good does a resistor do? > >Meindert >
You will have to ask those engineers who do so. Personally, with CMOS, I tie directly.
Meindert Sprang <mhsprang@NOcustomSPAMware.nl> says...
> ><uguess@nowhere.net> wrote...
>> Cmos is a high impeadance input that can be tied directly to either >> sink or source without resistors, but the general design standard is >> to use a single resistor to vcc as a "rail tie" for all high ties. > >Why?????
To make the board much, much harder to route. It's part of the PWB Layout Union's rules and is required by the PWB designer Full Employment Act of 1969 for all government contracts... :) -- Guy Macon, Electronics Engineer & Project Manager for hire. Remember Doc Brown from the _Back to the Future_ movies? Do you have an "impossible" engineering project that only someone like Doc Brown can solve? My resume is at http://www.guymacon.com/
On Sat, 10 Jul 2004 13:30:38 -0700, "Mike Turco"
<miketurco@yahoo-nospam4me.com> wrote in comp.arch.embedded:

> If I want to tie CMOS (74HCXX) pins hi or low, do I need to use resistors? > If I need resistors, can I connect pins together and then tie them hi/lo > through a single resistor?
You have had quite a few answers already, on both sides of the issue. With 74HC (but not 74HCT) it is not absolutely necessary as long as your power supply voltages will be fairly clean. And if they are too "unclean" you can have problems from the power supply pins anyway. On the other hand, the standards for many manufacturing organizations these days prohibit directly connecting power pins to either logic supply. They require a resistor in the 1K to 10K range so that they can drive it to the opposite level in automated test fixtures. -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html