74 series ic at 3.3 volts

[hock]

ic de 3.3volts

i think it is possible to use 5v 74 series ic in 3.3 volts designs provided al the devices are working on 3.3 volts. Has some one used it?
can some one comment why it should not be used in a all 3.3 v design

hock

 

[IanP]

74lvcxx

Standard 74 series will not tolerate Vcc below 4.7V.
For 3V applications you may use 74LV (low voltage), 74ALV, etc.
Folow this link for voltage thresholds for TTL series: http://www.interfacebus.com/voltage_threshold.html
Regards,
IanP

 

[hock]

the ttl compliance is valid only if the signals are 5 volt ttl.
if i have a standalone board where all the ic's are powered using 3.3 volts then in that case all the signals will be in 3.3 volt acceptance range.
ttl ic's should not have any power supply related issue for 3.3 volts devices.

 

[IanP]

All levels can be within 3.3V range but the problem lays in the supply voltage. Theses circuits were designed to operate of +5V supply and they can tolerate higher and/or lower voltages levels, but it can be unpredictable how a certain IC series will behave if supplied of any lower Vcc then specified (Vcc min=4.7V).
Regards,
IanP

 

[Bingo600]

You can not use a standard 74xxx at 3.3v , the function will be unpredictable as IanP says.

You can use a 74LVxx or a 74LVCxx and if you choose the TI 74LVCxxxA series then it is 5v tolerant on input , and can be used as a level translator.

/Bingo

 

[drwho78]

You can run standard 74HCXXX parts at either 5V or 3.3V.

 

[Aser]

The cardinal solution
is to implement the circuit in CPLD.
CPLD provides any logic level standard by any supply voltage under 3.3V.

 

[power-twq]

Because 5V device's speed is lower when they are operating at 3v3,

so, if you want a device operating at 3V3, you can selected 3V3 device directly,

TI has all what you want, 74LVC, 74LVT series logic is good for your needs.

i think it is possible to use 5v 74 series ic in 3.3 volts designs provided al the devices are working on 3.3 volts. Has some one used it?
can some one comment why it should not be used in a all 3.3 v design

hock

 

[hock]

The question was asked only from academic point of view. It is on the lines of what if questions, I could not see any thing in LS_TTL series data sheets which could convince me that I can not use it at 3.3 volts. That was the principle reason I had asked the question, I am fully aware of the low voltage series devices.
Thanks all for there response.
hock