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Original question:

SN74LVC1T45: Conversion between TTL 5V and LVTTL 3.3V

SN74LVC4245A: what bus transceiver to translation from a 3.3V to a 5V/3.3V system?

BrokenPin
Intellectual 410 points
Part Number: SN74LVC4245A
Other Parts Discussed in Thread: SN74LXC8T245, TMUX1574, SN74LV4066A

Hello,

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 the datasheet states that the control circuitry (DIR, OE) is powered by VCCA (i.e. 5V).
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, it will shorten 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,

  • +1
    Guru 317770 points

    The inputs have a very high impedance, so no current will flow. But 3.3 V signals are not enough for a 5 V input.

    Just use a more flexible device like the SN74LXC8T245 that allows you to connect the VCCA side to the MCU.

  • 0 in reply to Clemens Ladisch
    Intellectual 410 points

    Thanks for the reply Clemens,

    The good thing is that it is TSSOP package and the bad thing is that output enable and direction pins are referenced to vcca, if they were referenced to vccb, i would not change a thing in my design but.. it seems that I have to reroute the PCB board ..

    as I am going to interface the MCU I/O to a connector to be used as either serial communication protocols, digital I/O, or analog inputs... Do you think the SN74LXC8T245 will not affect the functionality of any of the use cases? As per the datasheet, the propagation delay, and current capabilities are good enough but I don't know about analog inputs!?

  • 0 in reply to BrokenPin
    Guru 317770 points

    Digital I/Os must not be used with analog signals; see [FAQ] How does a slow or floating input affect a CMOS device?

    To disconnect the transceiver from the analog signals, use switches like the SN74LV4066A or TMUX1574.