What you see
A sketch is ready to move from the browser editor to a supported physical board.
Likely causes
- Compilation, port access and the bootloader handshake are separate steps. Success in one step does not prove success in the next.
Diagnostic steps
Select the exact target
Choose the physical processor, bootloader and clock before Verify. Uno R3 is not Uno R4. Classic Nano has old and new bootloader profiles; Pro Mini has 5 V / 16 MHz and 3.3 V / 8 MHz variants. Newer upload profiles remain experimental pending physical validation.
Verify first
Fix the first actionable compiler diagnostic. Check Build Output for the application flash and static SRAM measurements; static SRAM excludes stack and heap. A compile failure does not mean the board disconnected.
Grant one port permission
Connect a USB data cable and press Connect board. Select the device yourself. If the picker is canceled, no new port is selected; that is not an upload failure. Close other programs that own the same port.
Upload and read the result
Press Upload and let the uploader perform its reset/bootloader protocol and firmware verification. Do not disconnect the cable during this operation. Port opening alone does not identify the MCU. Read the actual upload result before assuming code reached the board.
Inspect the running sketch
Open Serial Monitor at the sketch’s baud rate. Some boards reset when the port opens; allow setup() to run again. Use Serial Plotter for supported numeric series. Export the source when you finish.
Board-specific notes
Uno/Nano use their USB-to-serial bridge. Mega’s extra UARTs are not separate browser ports. Leonardo/Micro native USB may re-enumerate during reset. Pro Mini requires an external adapter with the correct logic voltage.
Safe recovery
If upload fails but the same port remains selected, inspect the error and board/bootloader setting before reconnecting. Avoid repeated resets while an operation is active. Stop or wait for it to finish first.
Evidence and references
Reviewed 2026-10-23 against Circuitera’s actual behavior. An editorial review is separate from a physical board test.
Circuitera implementation evidence
src/App.jsx: uploadSketch; src/serialConnection.js