LIBRARYDebugging a bus.

When a bus does nothing, the cause is almost always simple and physical: a missing pull-up, the wrong mode, two devices at one address, or a swapped wire. The fastest way to find it is a logic analyzer, which shows you the actual bits on the wire and turns a dead bus into a specific fault you can point at.

Check power and ground first

Before you suspect the protocol, confirm the boring things: both parts are powered, they share a common ground, and the wires go where you think they do. A missing ground between two boards is the single most common reason a bus is silent.

The usual suspects

Match the symptom to the bus. On I2C, no pull-ups means the lines never reach a valid high, so nothing moves, and two devices at the same address answer at once and collide. On SPI, the wrong mode (CPOL/CPHA) makes every byte read wrong even though the wiring is right. On UART, a baud mismatch prints garbage, and swapped TX and RX means nothing arrives at all.

The logic analyzer

A logic analyzer clips onto the bus lines, records their exact high-low timing, then decodes it into bytes, addresses, and acknowledgements. It shows whether the clock is even running, whether a device answered, and where the sequence broke. An inexpensive analyzer with a sigrok/PulseView setup is enough for most on-board buses.

1. Confirm power and a shared ground. Both parts powered, both grounds tied together, before anything else.
2. Check the wiring against the schematic. TX to RX, SDA to SDA, no swapped pair.
3. For I2C, confirm the pull-ups are present and no two devices share an address.
4. For SPI, confirm the mode (CPOL/CPHA) matches the peripheral's datasheet.
5. For UART, confirm both ends are set to the same baud rate.
6. Clip on a logic analyzer and read the actual bits: is the clock running, and did the device answer?

Bringing up a One Thousand Drones board, an analyzer on the I2C lines shows the controller sending an address and then either an acknowledgement from the sensor or silence, which tells you in a single capture whether the part is alive, mis-addressed, or missing its pull-ups.