• UART
  • Passive sniff
  • Serial CLI
  • 5 min

Recipe · Beginner · UART

How to sniff UART between two boards

A sensor board and a main board were talking during boot, but the useful text disappeared in two seconds. The goal was to listen without becoming a third device on the link.

UART signal lines between two connected devices.
UART sniffing is most useful when it stays boring: common ground, passive RX taps, no injected bytes.

Wiring View

UART link BP Side A TXUART RX GPIOSide B TXUART TX GPIOGNDBP GND
Generated from the wiring summary: UART link to BP.
Step 1

Find or set the UART format

The first time you enter UART mode, Bit Pirate asks for RX GPIO, TX GPIO, baud rate, data bits, parity, stop bits, and inversion. If you only have one active line, autobaud can listen on the configured RX GPIO until it detects a baud rate.

Step 2

Sniff the exchange

Use text mode for boot logs and AT-style protocols. Use raw mode when the stream looks binary or unreadable but you still need timing and byte patterns.

What the result means

Readable text is a big clue

If the output is readable and the directions are separated, you have enough to identify who speaks first, which baud rate is used, and which messages happen around reset. Garbled text usually points to baud, parity, inversion, or voltage mismatch.

Troubleshooting

The two common mistakes are swapped assumptions and impatient wiring. A board label named RX is usually the target's receive pin, so the other board's TX may be the useful signal. Also, if one side is idle during boot, the capture can look one-sided and still be correct.

  • Tapping a target RX pin instead of the other board's TX pin.
  • Using the wrong baud rate, parity, stop bits, or inversion setting.
  • Forgetting common ground, which makes readable text unlikely.
  • Connecting an active transmit line when the goal is passive sniffing.

UART sniffing FAQ

How do I sniff UART without interfering with the link?

Tap the existing TX lines into receive inputs and share ground. Do not drive the target lines while sniffing, and keep the setup passive unless you intentionally switch to a bridge or transmit workflow.

What can a one-sided UART capture still teach me?

A one-sided capture can reveal boot logs, baud rate, command vocabulary, framing style or which board initiates the exchange. It is still useful before you attempt active bridging.

Should I use text or raw mode for UART sniffing?

Use text mode for boot logs, console messages and AT-style traffic. Use raw mode when the stream contains binary frames, checksums, zero bytes or characters that make the text output look broken.

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