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[FAQ] Do I still need pull-up/pull-down resistors with bus-hold circuitry?

FAQ: Logic and Voltage Translation > Input Parameters >> Current FAQ

Short answer: External pull-up/down resistors are not recommended for devices with bus-hold circuitry. 


A pull-up or pull-down resistor will create a voltage-divider with the bus-hold circuitry any time the bus holder is in the opposite state to the resistor's supply.

For example:

With a typical bus-hold circuit, the output resistance is around 1kohm. If you have a 10kohm pull-down resistor at the input and are not otherwise actively driving the line, there are two states this can be in:

(1) The bus-hold circuit is in the low state, and there's no issue.

(2) The bus-hold circuit is in the high state, and now the input voltage is pulled down to ~0.9 * VCC.

#2 isn't much of an issue if you don't care what state the input is at, but if your intention of the pull-down is to put it in the low state, then you can see it won't work unless the resistor is very small (ie less than 1kohm).

It's much better to just not use bus-hold if that's the case.

You can read more in depth about bus-hold circuits and how they work in this application note: Implications of Slow or Floating CMOS Inputs in section 6.