Pullups for SPI voltage level translator

Projects Networking, Protocols, and Devices
1

Hi all,

I am using an 8-bit bidirectional voltage translator (TXB0108; http://www.ti.com/lit/ds/symlink/txb0108.pdf) to step down SPI signals from the Arduino to a display from 3.3V to 2.5V. My question is whether or not pullups are needed between the arduino and the voltage translator.

Here is the schematic of the translator:

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I have a pull-down on the output enable pin as suggested in the datasheet. My understanding is that when the outputs are disabled, all pins are in a high impedance state, to prevent excessive current drain/sourcing while the MCU initializes its outputs. Once the output is enabled, I'm a little less sure. My reasoning is that , because there are inputs on the "low voltage" side (nets marked with _L) - i.e. those on the display - pullups are needed to keep the pins in a definite state when they aren't actively being driven low. But I'm not sure about the MCU -> voltage translator. Should there be pullups on this line also?

2

The device cannot tolerate pullups or pulldowns unless very very weak (greater than 50k). It only drives weakly in the steady state, with strong drive only on transitions. Its a smart device.

So if you are worried about initialization time use 100k pull-ups / downs and enable the outputs once everything is set up. Otherwise just hard-wire the enable on and it should "just work".

3

FWIW, I do not use pullups on this device on the 'downstream' side (in your case 2.5V). I do use pullups (10ks) on the 'upstream' side for the SS pins to make programming the chip via SPI easier. HTH