SN54LVTH162244: Clarification on Bus-Hold

Jejomar Ildefonso

Part Number: SN54LVTH162244

Hi Team,

Need your expertise on this. We understand that input pull-up/-down resistors to SN54LVTH162244 are not recommended due to the bus-hold circuitry which puts the device on a valid logic state. But what are the possible implications of adding pull-up to the input of SN54LVTH162244?

Asking this as our customer is currently debugging a board using JTAG where the input signal going to SN54LVTH162244 is pulled high by a resistor to 3.3V Vcc. However, they ran across an output of 0.9V, which makes them wonder if the bus-hold circuitry is somehow being latched low.

Is it possible to put the SN54LVTH162244 input high by default? If so, what would be the recommended configuration for this? Would this be possible by choosing a resistance that can drive enough current to override the bus-hold?

Thanks in advance!

Kind Regards,

Jejomar

over 3 years ago

+1 Rami Mooti1 over 3 years ago

TI__Mastermind 28088 points

Hey Jejomar,

As you mention, adding an external pull up resistor is not recommended. This could create a voltage divider with the internal resistor and the external and could cause errors in the last known state of the device. I wouldn't recommend doing this, if you're expecting to get reliable results.

To learn more about the bus-hold circuitry take a look at this app note https://www.ti.com/lit/an/scla015b/scla015b.pdf

The default will be unknown for this. There isn't a configuration that would change this but it will have a 'tendency to be low'. This is different from a guarantee however
See [FAQ] What is the default output of a latched device? (Flip-Flop, latch, register)

You may be better off using a non bus hold device such as the SN54LVT16224B if the external pull up resistors are a must in your system.

Thanks,
Rami

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SN54LVTH162244: Clarification on Bus-Hold