Mixing logic families

vsmGuy · 01-08-2008, 11:13 AM

Guys,

Is it advisable to interface LS family digital logic parts with a uC like AT89C52 or PIC, or should I use HC parts instead ?

For example, would it be a good idea to connect a 74LS74 to the above mentioned uC pins or would it be better to connect a 74HC74 ?

An anecdotes and ideas/suggestions ?

**shadow** · 01-08-2008, 02:35 PM

I have not looked at those chips yet, however mixing logic families can give you alot of problems. The chips that you use have to be compatible with each other otherwise it is a real nightmare getting it to work the way you would want it. The easiest way of making sure that chips are compatible is to use chips from the same series from the same manufacturer if possible. That way you do not need to look at data sheets and deduce whether your chips will work with each other.

Having said this, the main aspects that you will be interested in the datasheet is the minimum voltage recognised as a 'high' and the maximum voltage recognised as a 'low'.

**gg1978** · 01-08-2008, 03:52 PM

what kind of voltage do you have on the output pins of the PIC or Atmel uC?? If they're 3.3V, i wouldn't use a LS series myself, but at 5V it should be fine. Technically speaking regular TTL "should" be OK at 3.3V, as logic high is anything between 2.0 and 5.0x V..

HC should work either way, as long as you're within voltage specs.. Also LVTTL is an option if your chip is 3.3V or less.. Not sure how low LVTTL can go..

Make sure if you use a CMOS based chip to tie unused inputs to either Vss or Vcc, or it's possible to damage them.. Or it was back when i was in electronics school anyway..

**PeteS in CA** · 01-08-2008, 04:14 PM

Their V(cc) have to match (74LS uses +5V). Beyond that, the 74LS has to be able to operate at the clock frequency for the uC. So you need to look at the max. operating frequency for the 74LS series.

vsmGuy · 01-09-2008, 05:36 AM

Quote:

Originally Posted by PeteS in CA

Their V(cc) have to match (74LS uses +5V). Beyond that, the 74LS has to be able to operate at the clock frequency for the uC. So you need to look at the max. operating frequency for the 74LS series.

Very nicely answered !

The ATMEL 89 series and PIC seem to have TTL compatible IO. Hence, LS should not be a problem.

I was sorry I did not mention this, because I had a mind block with the clock freq.

What I was querying about was whether anyone had speed sunc issues.

LS logic hit the max at ~25Mhz whereas HC at ~65Mhz, unless I am mistaken.

byteexp · 12-03-2008, 10:31 AM

i think there should be no problem mixing atmels or pic micros with other logic familys, LS are for low-speed circuit (4mhz?) while HC are for high speed circuits (20Mhz?)

it will be based on the design and purpose of the circuit.

01-08-2008, 11:13 AM	#1
vsmGuy Member Join Date: Jul 2006 City & State: southern california Posts: 28	Mixing logic families Guys, Is it advisable to interface LS family digital logic parts with a uC like AT89C52 or PIC, or should I use HC parts instead ? For example, would it be a good idea to connect a 74LS74 to the above mentioned uC pins or would it be better to connect a 74HC74 ? An anecdotes and ideas/suggestions ?

01-08-2008, 02:35 PM	#2
shadow Badcaps Veteran Join Date: Feb 2007 City & State: Melbourne, Victoria My Country: Australia Posts: 725	Re: Mixing logic families I have not looked at those chips yet, however mixing logic families can give you alot of problems. The chips that you use have to be compatible with each other otherwise it is a real nightmare getting it to work the way you would want it. The easiest way of making sure that chips are compatible is to use chips from the same series from the same manufacturer if possible. That way you do not need to look at data sheets and deduce whether your chips will work with each other. Having said this, the main aspects that you will be interested in the datasheet is the minimum voltage recognised as a 'high' and the maximum voltage recognised as a 'low'.

01-08-2008, 03:52 PM	#3
gg1978 Badcaps Veteran Join Date: Dec 2004 City & State: Jamestown, IN My Country: USA Line Voltage: 117VAC, 60Hz I'm a: Professional Tech Posts: 423	Re: Mixing logic families what kind of voltage do you have on the output pins of the PIC or Atmel uC?? If they're 3.3V, i wouldn't use a LS series myself, but at 5V it should be fine. Technically speaking regular TTL "should" be OK at 3.3V, as logic high is anything between 2.0 and 5.0x V.. HC should work either way, as long as you're within voltage specs.. Also LVTTL is an option if your chip is 3.3V or less.. Not sure how low LVTTL can go.. Make sure if you use a CMOS based chip to tie unused inputs to either Vss or Vcc, or it's possible to damage them.. Or it was back when i was in electronics school anyway.. Last edited by gg1978; 01-08-2008 at 03:55 PM..

01-08-2008, 04:14 PM	#4
PeteS in CA Badcaps Veteran Join Date: Aug 2005 City & State: San Jose, CA My Country: USA, Unsure of Planet Line Voltage: 120VAC, 60Hz & 115VAC, 400Hz I'm a: Professional Tech Posts: 3,385	Re: Mixing logic families Their V(cc) have to match (74LS uses +5V). Beyond that, the 74LS has to be able to operate at the clock frequency for the uC. So you need to look at the max. operating frequency for the 74LS series. __________________ PeteS in CA Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells. ************************** To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it. ************************** Anti-Covid-Vaxxer pig crap claim/prediction, Doctor: Heart Failure from mRNA Jabs "Will Kill Most People" \| Principia Scientific Intl. ; Dr. Geert Vanden Bossche Warns COVID-19 Jab Injuries and Deaths Will Soon "Collapse Our Health System" (VIDEO) ; Fully Vaxxed May 2021; Since that time I've done 7 5Ks, 1 8K, 8 10Ks, and 4 half marathons

12-03-2008, 10:31 AM	#6
byteexp New Member Join Date: Oct 2006 Posts: 10	Re: Mixing logic families i think there should be no problem mixing atmels or pic micros with other logic familys, LS are for low-speed circuit (4mhz?) while HC are for high speed circuits (20Mhz?) it will be based on the design and purpose of the circuit.

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