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  1. #1
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    TTL Logic gate IC - 74HCxx vs 74LVxx

    Dear all,

    I found this useful information from TI.


    http://focus.ti.com/lit/an/scba011/scba011.pdf

    But i still have doubt. May i know those 74HC573, 74HC245, 74HC02, 74HC32, 74HC138, and so on..Is CMOS or TTL IC? If those 74HCxx is CMOS IC then i will facing problem. Analyze as bellow:..


    Q1)
    5.0V CMOS Voh=4.44V
    3.3V LV Vih=2.0V
    should not be any problem because Voh (CMOS) > Vih (LV), am I rite? In this case the 5V signal will harm the 3.3V device?
    Q2)
    5.0V CMOS Vol=0.5V
    3.3V LV Vil=0.8V
    should not be any problem because Vol (CMOS) < Vil (LV), am I rite?



    Q3)
    3.3V LVC Voh=2.4V
    5V CMOS Vih=3.5V
    Q4)
    problem because Voh (LV) < Vil (CMOS), am I rite?
    3.3V LVC Vol=0.8V
    5V CMOS Vil=1.5V
    should not be any problem because Vol (LV) < Vil (CMOS), am I rite?


    I very confusing after compare the TI information with 74HCxx datasheet. I did check the datasheet for 74HC245. The datasheet shown bellow:

    Q5)
    74HC245 is CMOS device but the voltage level is not like TI information mention. This device voltage level most like TTL and LV category, am i rite?


    One more doubt. Hope can get some advice.
    All my board device are using 5V operation and almost all the logic gate IC is 74HCxxx. I have one LCD which is operate at 3.3V.
    Q6)
    Can regulate the supply from 5Vcc to 3.3Vcc for all the 74HCxxx? make it standard voltage in the system. So that all the 74HCxxx Vil, Vih, Vol and Voh signal can be matching. I did check 74HC138, 74HC32, 74HC02 and so on... their min operation voltage is 2Vcc.
    Q7)
    Some of the device i can't use 3.3V like example i have using 82C55 I/O expander, 4Vcc min operation, 62256 SRAM and so on... If the board able to drive 3.3V devices then 5V devices might not be drive-up. Any simple way or opinion can be advice?

    Thanks...
    Last edited by Help; 19th April 2011 at 03:39.

    •   Alt18th April 2011, 20:12

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  2. #2
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    Re: TTL Logic gate IC - 74HCxx vs 74LVxx

    Hi
    Use Level converters.
    If ur problem is not solved then please express in words again.



    •   Alt18th April 2011, 20:55

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    Last edited by kak111; 18th April 2011 at 22:31.


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    •   Alt18th April 2011, 21:38

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    Re: TTL Logic gate IC - 74HCxx vs 74LVxx

    1) 74HCxx are High speed CMOS and mostly they are pin compatible with 74xx, if not then use 74HCTxx which High Speed CMOS TTL pin compatible.
    2) Never use ICs from different Power levels they may work in the bread board test but will malfunction during operation.
    3) 74HCxx can be used with 74xx or 74LSxx, or any (5V power)
    4) i have used 74HCxx, ADC (5V), 82C55 and microcontroller all on one board with 5V.


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    Re: TTL Logic gate IC - 74HCxx vs 74LVxx

    Dear All,

    Thanks for you all reply. Now i have some idea after the suggestion and research. I found one article which is very useful and some of the answer i can get it from there. Please have a look: I've got a question for you. - SparkFun Electronics

    Ya, may be some of the part i should use 74HCTxx and 74LVCxx as well.

    74LVCxx have this 2 following feature:
    Operates 1.65V-3.6V;
    Inputs Accept Voltage 5.5V;

    but some of the logic-gate don't have 74LVC series like example 74HC30. I wonder is there any replacement for that 74HC30 NOR-gate IC.

    Dear mfarooq1:
    Do you tried 3.3V data bus line for your ADC, and 82C55 before?

    Thank you..



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    Re: TTL Logic gate IC - 74HCxx vs 74LVxx

    No, my system was operating on 5 V logic.


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