I have figured it out later. UARTx_FIFO_REG has to be written into using sw instruction instead of sb

In esp32c3 technical reference 26.4.2, there is: UARTn can access FIFO via register UART_FIFO_REG. You can put data into TX FIFO by writing UART_RXFIFO_RD_BYTE, and get data in RX FIFO by reading UART_RXFIFO_RD_BYTE. If I get it right, it means that reading the lower 8 bits of UART_FIFO_REG will rea...