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Old 01-08-2008, 11:13 AM   #1
vsmGuy
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Question 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 ?
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Old 01-08-2008, 02:35 PM   #2
shadow
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Default 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'.
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Old 01-08-2008, 03:52 PM   #3
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Default 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..
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Old 01-08-2008, 04:14 PM   #4
PeteS in CA
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Default 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.
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Old 01-09-2008, 05:36 AM   #5
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Thumbs up Re: Mixing logic families

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.
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Old 12-03-2008, 10:31 AM   #6
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Default 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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