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Original question:

SN74LVC8T245: 5VTTL to 3.3VTTL bidirectional level shifter: What's the Best choice

SN74LVCC3245A: Interfacing with 5V TTL signals with dynamic bidirectional communication from a 3.3V microcontroller

Dave Young
Intellectual 520 points
Part Number: SN74LVCC3245A
Other Parts Discussed in Thread: SN74AHC245, SN74LVC8T245, SN74LV126A

Hi there-

I'm trying to take a 3.3V microcontroller and communicate with some very old legacy 5V TTL signals. Initially, I was tempted to use the SN74LVCC3245A but I noticed through another forum post that it must have a HI level input voltage minimum of 3.5V.

I was able to find some uni-directional level shifters and/or buffers that are able to act as either a bank of inputs or outputs, but cannot be switched. This application requires re-configuring the part to be bi-directional via the microcontroller (using a direction pin) since there are many different confgurations that the system needs to accommodate.

Given these requirements, would you have a part to suggest or perhaps a tweak to the SN74LVCC3245A circuit that would allow it to accept these TTL signals without being overly complex or expensive?

Thanks!

Dave

  • 0
    Guru 257000 points

    Are the legacy signals 5 V TTL or 5 V CMOS?

    Because in the former case, 3.3 V is a valid high voltage, and you can simply run the transceiver with overvoltage-tolerant inputs at 3.3 V (and use a simpler one like the SN74AHC245).

  • 0 in reply to Clemens Ladisch
    Intellectual 520 points

    Thank you for the speedy reply!

    The 3.3V-side is CMOS from a microcontroller which needs to both receive inputs from the 5V TTL legacy signal (an acceptable HI level from the signal source could have a minimum of 2V), and with a flip of a direction bit, output to a 5V CMOS signal (output voltage must be greater than 3.5V, ideally greater than 4.95V). I know it's obnoxious to have to cover all of these cases, but I'm trying to design around industrial systems that could be installed 30 years ago (TTL) and also those that were designed new this past year (CMOS).

    I see what you're saying, powering the SN74AHC245 at 3.3V and if it gets hit with an input signal of 5V it doesn't care. However when the direction is flipped the other way it will only output a 3.3V signal which isn't enough for a 5V CMOS system to see a HI.

    would there be a part (or couple of parts) that can be used for bi-directional communication between a 3.3V microcontroller and a 5V system that might be either CMOS or TTL?

  • 0 in reply to Dave Young
    Guru 257000 points

    There is no direction-controlled level shifter with TTL-compatible inputs.

    So you have to use two devices. For the 3.3 V to 5 V direction, use a normal shifter (e.g., SN74LVC8T245). For the 5V to 3.3 V direction, use any buffer running at 3.3 V with overvoltage-tolerant inputs and with active-high OE pin (e.g., 2× SN74LV126A).