When nothing crosses
Four readings with a meter, black probe on GND, tell the usual faults apart: 5V on the header's 5V pin, 3.3 V on the socket's 3V3, 3.3 V on your board's L pin at rest and 5 V on the part's H pin at rest. A 3.3 V board's pin reading well over 3.3 V means it is on the wrong row. Readings that are all right and a signal that still does not cross mean the channel numbers do not match.
Four readings
Power the board with nothing driving the channel: no sketch running, or a sketch that leaves the pin as an input. Put the black probe on GND, and take four readings.
- The header's 5V pin. 5 V, give or take a few tenths on USB.
- The socket's 3V3 pin. 3.23 to 3.37 V, the ME6211's 2 % either side of 3.3 V.
- The L pin your 3.3 V board is on. 3.3 V, from its 10 kΩ.
- The H pin the 5 V part is on. 5 V, from its 10 kΩ.
What each wrong reading means
Pick each wiring in the figure.
- All four at zero: no 5V. The regulator has nothing to work from, so the 3.3 V side is dead too. This is the most common one.
- Your board's pin well over 3.3 V: it is on the header's row, not the socket's, and the H pin's 10 kΩ is pushing 5 V at it. That is the one reading that can damage something. Unplug it first.
- All four right and nothing crosses: the two ends are on different channels. The meter cannot see it; the numbers printed on the board can.
Then the loopback
When the four readings are right, the loopback sketch checks the rest: two channels, both directions, with one jumper. If that passes and your part still does not answer, the TK97 is working and the fault is in the part, its library or a pin number in the sketch.
When it does not work
5V is not reaching the header, or GND is not shared. Measure your board's own 5V pin against its GND: if that is 5 V, the wire between them and the TK97 is the fault. On a Pico, VBUS is 5 V only while it is on USB.
Something is feeding it from outside. The regulator cannot make more than about 3.37 V from 5 V. Unplug everything from the socket and measure again; a block or a wire that brought 5 V into the socket is the cause.
It is on an H pin. Each H pin has 10 kΩ up to 5V, which pushes current into a 3.3 V input through its protection diodes; the reading lands between about 3.6 and 5 V depending on your board. Move it to the socket's row before anything else.
Check that your pin's L number matches the part's H number. A 3.3 V board on L1 and a part on H2 are on two different channels, and a meter at rest cannot show it. The loopback sketch reports it as always HIGH.
A USB supply under load, or a long cable. The regulator still gives 3.3 V from anything above about 3.42 V at 100 mA, so this matters only if the part on the 5 V side needs a full 5 V.
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