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Original question:

TXU0304: Propagation Delay Time

TXU0304: TXU0304 for SPI, speed vs propagation delay

Jesper Svanfeldt
Prodigy 10 points
Part Number: TXU0304
Other Parts Discussed in Thread: SN74AXC4T774

I'm looking into use the TXU0304 in an SPI application converting from 1.8V to 2.5-3.3V. Speed of SCK is 32MHz.  At a first glance the TXU0304 should be suitable, but when investigating the propagation delay one can see that it's broadly specified 0.5-20ns (approx). With worst case 20ns there will be a time-difference between SCK and MISO (slave-out) of more than 40ns at the master (2*propagation delay in TXU0304 + set-up time for slave) and therefore you cannot use the TXU0304 for such high speeds?!

Do you have any solution or suggestion for solving this? 

  • 0
    TI__Mastermind 43156 points

    Hi Jesper,

    Yes, propagation delay may be a rough estimate for speed. However, please note that the estimate will be lower than what the devices can achieve.

    Please see below as 32 MHz is plenty of margin for the device, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 10 points

    You miss the point that for SPI you must consider the propagation dealay twice (once for the SCK going from master to slave, and once for the MISO going from the slave to master through the level translator.) Plus adding to this is the setup time for the slave. 

    The datarate you refer to is only valid for the MOSI where the issue is skew between SCK and MOSI. In this case 8ns. 1/8e-9 = 125E6. 

  • 0 in reply to Jesper Svanfeldt
    TI__Mastermind 43156 points

    Hi Jesper,

    Thanks for clarifying.

    Jesper Svanfeldt said:
    The datarate you refer to is only valid for the MOSI where the issue is skew between SCK and MOSI. In this case 8ns. 1/8e-9 = 125E6. 

    Note that the skew highlighted in the datasheet is the difference between two output channels of the translator while the propagation delay is between an input and an output.

    Please also see SN74AXC4T774 for faster propagation delay, thanks.

    Best Regards,

    Michael.