modelLoopback

Information

Demonstrates the machine.UART serial link as a real electrical signal: the TX pin carries a real frame (start bit at 0, 8 data bits least significant first, stop bit at 1), each bit lasting 1/baudrate. Plotting mcu.GP0.v lets you read the frame by eye in OMEdit, as on an oscilloscope.

The script waits 5 ms before transmitting, to show the idle state on the plot: as soon as the UART is configured, the TX pin is actively driven high ("mark" state), even before the first write(). The TX holds the line, not the RX — the output is push-pull, no pull-up resistor is involved (unlike an open-drain I²C bus).

The waveform is produced by the C runtime, which publishes the line level and only requests a sync point at its changes, without the Python thread driving each edge — like the real UART peripheral of the RP2040, which runs independently of the CPU once programmed (see requirements.md). Reception is decoded on the C side from the edges of the line, with the same mid-bit sampling as a real receiver.

The TX→RX loopback must reuse the loopR/loopC pattern of Examples.Gpio.PinEcho: connecting two pins of the same MCU with a direct connect() makes the driven voltage disappear from the simulation results.

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