8 November 2026 · Soldr
avrdude: stk500_getsync() not in sync: fixing Uno and Nano uploads
Short answer: the upload tool couldn't talk to the bootloader on your board. In order of likelihood: the wrong port is selected; a compatible Nano needs the "ATmega328P (Old Bootloader)" processor option; something is connected to D0 or D1 (the serial pins the upload uses); the USB driver or cable is wrong; or the wrong board is selected. Work down the list.
On an Uno or a Nano, uploading goes through a small bootloader on the ATmega328P, talking over the board's serial port. "Not in sync" means the tool sent its hello and got no sensible answer. The good news: the cause is almost always a setting or a wire, not a damaged board.
1. The wrong port
Unplug the board and see which port disappears from the list, then plug it back in and pick that one. If no port appears at all, it's the driver or the cable. See CH340 and CP210x drivers. Compatible boards such as our Uno R3 (CH340G) and Nano R3 (CH340) use a CH340 chip that some computers need a driver for.
2. A Nano with the old bootloader
Many compatible Nanos ship with the older bootloader, which talks at a different speed. If you've selected "Arduino Nano" with the default processor, the tool and the board never sync. Choose Processor: ATmega328P (Old Bootloader) and upload again. It's the most common fix for Nanos, and it's covered in original vs compatible Arduino boards.
3. Something connected to D0 or D1
The official Uno R3 pinout marks D0 as RX and D1 as TX: the serial port the upload uses. A Bluetooth module, a GPS or any other serial device on those pins talks over the upload, and the bootloader never hears the tool clearly. Disconnect anything on D0 and D1 while uploading, or move it to other pins and use a software serial port in your code.
4. The wrong board selected
Select the board you actually have: "Arduino Uno" for an Uno, "Arduino Nano" (plus the right processor) for a Nano. An Uno R4 or a Mega is a different board with a different upload method.
5. The cable, the socket, the reset
- Use a data cable. A charge-only cable powers the board but carries no data.
- Plug in directly, not through a hub.
- Close other programs that might hold the port open, such as a second IDE window with a Serial Monitor open.
- Try a manual reset: start the upload and press the board's reset button just as the tool begins to connect. The bootloader only listens for a moment after reset.
6. Power and the board itself
A board powered from the USB port alone should upload. If the board's power LED is off, or the board is very hot, stop and check the wiring for a short before anything else. The Uno R3's official datasheet gives 20 mA as the maximum per I/O pin, and pins overloaded by a motor or a big LED load can make the whole board misbehave.
Uploading from the browser
Soldr compiles on its server and flashes Uno, Nano and Mega boards from Chrome or Edge on a computer, with no IDE to install. The same rules apply: the right board, the old bootloader option for many Nanos (Soldr supports the old-bootloader Nano in the browser; a new-bootloader Nano may fail there), nothing on D0 and D1, and a working driver. When it's set up, the upload is a few clicks.
On Compoden: Uno R3 (CH340G), Nano R3 (CH340), the original Arduino UNO R3. Describe your build on app.soldr.ai. Sign in and get 100 credits free.