EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: gogoman on August 31, 2023, 02:30:31 am
-
Hello the 74HCXX family is rated to operate with a VCC between 2 and 5V.
because its, considered a 5V device, Is there any reason not to use the device at 3.3v?
What are the limitations?
what are the voltage input high and low at 3.3V ?
what are the voltage output high and low at 3.3V ?
thanks
-
All of that information will be in the datasheet under "DC Characteristics".
-
At 3.3 volts, 74HC logic is slower because the channel resistance of the CMOS transistors is higher. The input threshold voltage is half way between the ground and supply voltages, so 1.65 volts.
-
There are the 74HC and the 74HCT, both are CMOS devices with a pinout compatible with the initial TTL family (Transistor Transistor Logic). The last T in the 74HCT series means the logic levels are compatible with the voltage logic levels used in the old TTL series.
CMOS digital circuits, by default have the logic level variable, it's at the half of their supply voltage, so it depends of the voltage you are using to power them.
As a thumb rule, any CMOS will work faster at a higher voltage. By how much faster it depends with each chip, you'll need to consult the datasheet.
-
Lower supply voltage also mean higher output resistance and thus less capable to drive loads (e.g. LEDs, relays).
The lower speed with a reduced supply voltage also has its good sides: usually less EMI produced.
The CMOS chips have the threahold usually at 30 and 70% of the supply. The range inbetween is undefined and can lead to significant higher supply current. Even the 74HCT chips may take more current with a 3 V input signal, even though in the range of defined high for this series.
-
Lower supply voltage also mean higher output resistance and thus less capable to drive loads (e.g. LEDs, relays).
The lower speed with a reduced supply voltage also has its good sides: usually less EMI produced.
The CMOS chips have the threahold usually at 30 and 70% of the supply. The range inbetween is undefined and can lead to significant higher supply current. Even the 74HCT chips may take more current with a 3 V input signal, even though in the range of defined high for this series.
the HCT usually specify that extra current, and HCT are only specified for 5V
-
74HC is perfectly suitable for 3.3V operation for general purpose applications. (You didn't say what, so we cannot possibly tell whether your particular application will actually work at all.)
If you need faster speed, there's also 74AC, 74LVC, and friends. Or 74ABT and etc. particularly for lower voltages (3.3 is on the upper end of nominal for them I think?).
Beware of signal quality concerns and switching noise emissions, particularly from fast families. Preferably, only use what speed grade you require.
Tim
-
My super fast 245-F :D
F = fast, what else?
-
P.s. I used an hct device to interface Sony psx pad with an fpga.
(The PAD is a removable device on an hot plug connector, so .. in my head I feel better to put an 245hct to protect the fpga)
It worked perfectly at 3.3v!
-
Why not just look at any data sheet?
Almost all of the data sheets, the good old ones from Motorola, TI, Philips, ect, all specify exactly what happens with their 74HCxxx's and 74HCTxxx's input and output thresholds, current drive and propagation delays with Vcc at 5.0v, 3.3v, 2.5v at a bare minimum.
-
My super fast 245-F :D
F = fast, what else?
F = FAST = Fast Advanced Schottky TTL, which is roughly comparable to AS or Advanced Schottky.
FAST had a pretty amazing combination of low supply current, high speed, and high output drive. I wish more of the TTL library of functions was made available in FAST.
-
Why not just look at any data sheet?
Almost all of the data sheets, the good old ones from Motorola, TI, Philips, ect, all specify exactly what happens with their 74HCxxx's and 74HCTxxx's input and output thresholds, current drive and propagation delays with Vcc at 5.0v, 3.3v, 2.5v at a bare minimum.
Modern data sheets that have the characteristics specified at -10% nominal VCC can be a better way.