When nothing happens
Four readings, in an order where each one only matters if the one before it passed. The black probe goes on the square pad and stays there, and the second reading tells you whether the problem is even on this board.
Black probe on the square pad
Every reading below is a voltage, and a voltage is meaningless without saying what it is measured against. Put the black lead on the square pad — pin 1, GND — and leave it there for all four readings. Moving it is how people end up measuring one board's 5 V against another board's idea of zero.
The order, and why it is this order
1. GND. Not a reading, a decision. This is the reference for everything that follows.
2. VBUS. This is the one that splits the problem in half.
- 0 V — the handshake did not happen. The fault is the cable, the source, or the board. Keep looking here.
- 5 V — the handshake worked, the source is supplying, and this board has done everything it does. The fault is downstream, and nothing else you measure on the breakout will help.
That second case is worth taking seriously. A board with 5 V on VBUS and a lit LED is a board with nothing wrong with it.
3. CC1 and 4. CC2. One of these sits somewhere between about 0.4 V and 1.7 V; the other sits at zero. That is correct and is not a fault — a cable connects one CC line and leaves the other open, and which one depends on the orientation of the plug. Turn the plug over and the readings swap.
What would be wrong is both at zero with VBUS live, or both showing the same non-zero voltage. The first suggests a bridge to ground; the second suggests CC1 and CC2 are joined somewhere, which puts the two resistors in parallel and gives the source a value it is not looking for.
The three cables in your drawer
More faults here are cables than boards.
- USB-A to USB-C. No CC wire exists at the A end, so there is no handshake to have. Many of these still deliver 5 V, because a USB-A port is always live — which is why this cable can make a board look fine when it is not, and make the CC readings look broken when they are not.
- Charge-only USB-C. Power but no data pair. Fine if you only want VBUS, useless the moment you wire D+ and D−.
- A proper C-to-C cable from a source with a C socket. This is the one the whole board is designed around, and the one to test with.
When it is the board
Rare, but it happens, and it is nearly always a joint you made rather than one the factory made. The suspects, in order: a bridge between two of the six pads, a header pin that looks soldered and is not wetted through, and heat damage from working too close to the connector.
Everything on this board that matters electrically — the socket, both CC resistors, the LED — arrives already soldered and tested. The only new joints are the six you added.
When it does not work
The handshake never happened, so the source never switched on. Try a different cable first — a USB-A to USB-C cable has no CC wire and a charge-only cable may have no data pair, and both are common in a drawer. Then try a different source. If a known-good C-to-C cable and a known-good charger still give nothing, look for a solder bridge between CC1 or CC2 and ground, which would put the pull-down at zero ohms instead of 5.1 kΩ.
Then the board has done its entire job and the fault is downstream. Check the ground actually reaches your circuit — a board that is powered through VBUS but grounded somewhere else is the classic version of this. Measure at the far end of your wiring rather than at the breakout, because what you are looking for is a volt going missing in a breadboard contact.
You are drawing more than the source offers or more than the wiring carries. Measure with the load running, at the board and then at the circuit. A big difference between the two readings is your jumper wires; a low reading at both is the source. Read what the charger offers will tell you which of the two you are up against.
Sources differ in how strictly they check CC and how much they offer. A lenient source will supply 5 V to almost anything; a strict one will not. If it works on a power bank and fails on a good charger, believe the strict one — it is telling you something about your wiring that the lenient one is hiding.
Adding up what a circuit really draws, so the next supply you choose is chosen rather than hoped for.
Working out a power budget →Edit this page — content/books/usb-c-breakout/when-nothing-happens.mdx
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USB-C Breakout Board, 12-Pack with 5.1 kΩ CC Resistors
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