Hello guys, so I am asking this here becouse I wasn't able to find this anywhere else.
So I have a 74hc161 chip running at 5V that I need to connect into another 74hc157 chip running at 3.3V.
I know that this probably requires level shifting, but the datasheet for 74hc157 doesn't specify the maximum input voltage on a pin.
So what is the maximum input voltage for 74hc? Is it Vcc or the maximum supply voltage (6V)?

 

The Vcc of 74HCxx series ic is 2~6V, but when you apply 3.3V for it then the input voltage better not to over 3.3V, otherwise it could be damaged, you can check the page 4 of 74HC04.

So you need to use the voltage shifter as 74HC06 or others and adding the pull up 10K resistor, please to check the page 2 has internal structure to shown that it is a open collector, adding +5V to the Vcc of 74HC06 and the adding the pull resistor to the 3.3V.

 

If you drive 5V into an input pin on a chip running at 3.3V you are going to drive current through the input protection diodes into Vcc of the lower voltage device.

Use a level shifter such as 74LVC245.

 

This is what the Nexperia datasheet for the 74HC157 says :-
"Input clamping current, for Vin < -0.5 V or Vin > V CC + 0.5 V, is ±20 mA maximum".
In other words, you can theoretically apply more than 3.8V to your '157 (with its 3.3V supply) providing you limit the current through the input protection diodes to a good bit less than 20mA. In practice, it would be better to use a level-shifter as has been suggested above.

 

Thanks alot for your replys, these were very useful. I am definetly going to look into some level shifters than.

 

If the operating frequency isn't too high, you could just put a resistor in series with the input to limit the current to a safe level.

 

Another option is to use 74HC4050.

 

If the frequency of operation isn't too high, you can make a resistive voltage divider and then you don't have to worry about turning on the protection diodes.

 

If the frequency of operation isn't too high, you can make a resistive voltage divider and then you don't have to worry about turning on the protection diodes.

Yeah I tought about that, but the 74lvc245 has very conveniently placed inputs on one side and outputs on the other side,
so I am going to use that. Plus I am working with 20MHz on some adress lines so I don't want to use resistors.
Thanks for your suggestion though.