All of USB, I2C, low power UART (9600 fixed), and high speed USART.
They all equivalently are JSON request/response ports. I2c uses a simple serial-over-I2C protocol.
Most customers use the I2C or low power UART interfaces because the device only uses ~8uA when listening on those ports. (Our internal MCU can listen on those ports while in STOP2 mode.)
