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SN74AXC8T245: Compatibility with Xilinx Zynq Ultrascale FPGA

RajK
Intellectual 740 points
Part Number: SN74AXC8T245
Other Parts Discussed in Thread: TXB0108, SN74LVC8T245, TXS0102

Hello,

We are implementing a design where the FPGA output is 1.8V and the peripheral inputs are 3.3V. We are looking in to potentially using SN74AXC8T245 as a level translator between the FPGA and peripherals. One SN74AXC8T245 as 1.8V to 3.3V and another SN74AXC8T245 as 3.3V to 1.8V.

The peripherals are low speed (1MHz - 70MHz) and most have either a pull up or pull down external resistor at the input. Some other have 100 Ohm resistors at the input. See attached photos below -






The trace lengths from SN74AXC8T245 to 3.3V peripherals are on an average 9000mils and the trace lengths from FPGA to SN74AXC8T245 are about 5000mils. 

While reading in the forum about TXB0108 and SN74LVC8T245, I learned TXB0108 does not have enough drive strength and SN74LVC8T245 is not optimized for 1.8V <--> 3.3V translation. Could you please recommend a part for this implementation. Please let me know if I can provide more information on the design under consideration. Thank you.

  • 0
    Guru 299450 points

    In general, a higher supply voltage for the same device result in higher drive strength.

    CMOS inputs have a very high impedance, so pull-up/-down or series resistors are no problem.

    LVC is optimized for 3.3 V; AXC is optimized for 1.8 V.

    For more than 8 signals, your choice of 2× SN74AXC8T245 is correct.

  • 0 in reply to Clemens Ladisch
    Intellectual 740 points

    Hi Clemens,

    Thank you for your response.

    Use LVC for 1.8V to 3.3V and AXC for 3.3V to 1.8V. Is this understanding correct ?


  • 0 in reply to RajK
    Guru 299450 points

    At 3.3 V, LVC has higher drive strength (24 mA) than AXC (12 mA). But if you do not need such a high drive strength, then you can simplify your BOM by using AXC for both directions.

  • 0 in reply to Clemens Ladisch
    Intellectual 740 points

    From the datasheets of LVC and AXC, I learned these parts do not support auto direction sensing. There are some signals, however, that are bidirectional. For example, the "J30_SDA" signal in the photo above.  I understand setting the "DIR" pin of  LVC or AXC  will change the direction of the signals. We are looking for a solution that does not require modifying the "DIR" pin during runtime. A desired solution is to use LVC/AXC  for unidirectional signals and hardwire the DIR pin and use a separate level translator for bidirectional signals like "J30_SDA". Could you please suggest parts for such bidirectional signals and confirm my understanding of the use of LVC/AXC. Thank you

  • 0 in reply to RajK
    Guru 299450 points

    For I²C, use the TXS0102 (it does not need external pull-up resistors).

  • 0 in reply to Clemens Ladisch
    Intellectual 740 points

    Thank you for the recommendation. When using TXS0102 for 1.8V <--> 3.3V, where should OE pin be connected ? Should it be connected to 1.8V or 3.3V ?

  • 0 in reply to RajK
    Guru 299450 points

    It does not matter.