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SN74AXC4T774: Multiple SN74AXC4T774PWR on the same SPI bus

Scott McDonald
Prodigy 20 points

Part Number: SN74AXC4T774

Hi
 
I was hoping you could help with a question that I have with the TI SN74AXC4T774PWR device?
 
In my project I have x2 SPI slave devices which operate at 2.5v.  The SPI master is a 1.8v device.  I plan on using the SN74AXC4T774PWR to act as a level shifter for the voltages.  My plan is to use x2 SN74AXC4T774PWR one for each SPI slave device.  Plan on using x2 because x2 would be needed anyway with the CS signal.
 
For development purposes I need to connect a USB to SPI debugger to the same SPI bus.  This device is 5v but also works at 3.3v.  This device allows me to read and write to the SPI bus.
 
I have had a PCB made which has x2 SN74AXC4T774PWR devices present which interface with the x2 SPI slaves.  During the testing of the board I realised that the USB/SPI debugger needs a higher voltage therefore my as made PCB has only x2 SN74AXC4T774PWR .  As is typically (for me) there are various issues with the board and I just want to rule out that having 2 or 3 SN74AXC4T774PWR devices working together on the same SPI bus should not result in any issues.  I cant see that it should cause any issues bSPI level converter diagram.pdf ut I just want to double check.
 
I have created a simple diagram of the SPI bus showing the master/salves and USB debugger.
  • 0
    TI__Mastermind 39760 points

    Hello Scott,

    The issue you may be experiencing could be bus contention. The SN74AXC4T774 has integrated input pull-down resistors meaning if you leave the inputs floating, the output will always send a low signal.

    For example, if SPI Slave 1 is sending MISO to the SPI Master, this line will be pulled low by both SPI Slave 2 and Aardbark level translators (even if both are idle). If this is the case then I would recommend forcing /OE pin high on all level translators not in use. 

    If bus contention is not the issue, then I would recommend probing the SPI bus to figure out why both AXC4Ts cannot work together.

    Regards,

    Josh