When Your Level Shifter Is Too Smart To Function

Estimated reading time: 2 minutes

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By now, 3.3V has become a comfortable and common logic level for basically anything you might be hacking. However, sometimes, you still need to interface your GPIOs with devices that are 5 V, 1.8 V, or something even less common like 2.5 V. At this point, you might stumble upon autosensing level shifters, like the TXB010x series Texas Instruments produces, and decide that they’re perfect — no need to worry about pin direction or bother with pullups. Just wire up your GPIOs and the two voltage rails you’re good to go. [Joshua0] warns us, however, that not everything is hunky dory in the automagic shifting world.

During board bring-up and multimeter probing, he found that the 1.8 V-shifted RESET signal went down to 1.0V — and its 3.3 V counterpart stayed at 2.6V. Was it a current fight between GPIOs? A faulty connection? Voltage rail instability? It got more confusing as the debugging session uncovered the shifting operating normally as soon as the test points involved were probed with the multimeter in a certain order. After re-reading the datasheet and spotting a note about reflection sensitivity, [Joshua0] realized he should try and probe the signals with a high-speed logic analyzer instead.

At a high enough frequency, he’s found the signals constantly oscillating back and forth, as the shifter’s autosensing mechanism was being fooled into switching by the signal reflections at a fast enough rate to confuse the multimeter into reading the signals as being at an in-between voltage. It turns out that even with signals that are meant to change only once in a board’s bootup, these shifters might give you more trouble than they’re worth. Not to worry, however, as you still have myriad ways to level shift any signal you want.

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