Hello guys,

I need to interface my 3.3V MCU IO ports to a connector that will be connected to either a 5V or a 3.3V system.
I learned about the SN74LVC4245A, an 8-bit bus transceiver with two separate supply rails VCCA(5V) and VCCB(3.3V). but in the datasheet, it states that the control circuitry (DIR, OE) is powered by VCCA.
My question is, can I connect (DIR, OE) to my 3.3V MCU pins since the MCU is the source of the I/O signals? I am afraid that if I connect them, they will short the 5V and 3.3V rails.!!
And if there are any other similar chips that translate 3.3V I/O to both 3.3V/5V systems?

Thank you,

 

This may not help, but depending on the 3.3V and 5.0V devices you are planning to connect together you may find that the 5V device will see 3.3V outputs as high, whilst you should connect outputs from the 5V device to the inputs of the 3.3V device via a potential divider. This certainly works for Arduino devices for example. In other words, a level shifting IC may not be necessary.

 

Technically OE and DIR are powered by whatever you connect it to so it shouldn't cause any issues with the 3.3 V. The reason they say to tie them to VCCA is so all voltages on the control ramp up the same as VCCA when power is connected to avoid issues.

You could connect a pull up resistor to both OE and DIR and use a transistor to pull them to low level.

Sorry for the mediocre drawing... it's quicker for me to do it by hand.

Edit:

I think the part in the datasheet about control circuits powered by VCCA is the internal circuits. The actual pin inputs are separate since it also mentions a pull up resistor on OE and can technically be run off 3.3 V, but they recommend VCCA.. It looks like 2 V is the minimum for a high level on the "A" side.

 

Welcome to AAC!

And if there are any other similar chips that translate 3.3V I/O to both 3.3V/5V systems?

What frequency?

For lower frequencies, you can use a MOSFET with 2 resistors (you can find these for much less than $3.50):



For higher speeds, you can use: