static void on_uart_rx_data(void *user_data, uint8_t byte) {
  // `byte` is the byte received on the "RX" pin
}

static uint8_t on_uart_write_done(void *user_data) {
  // You can write the chunk of data to transmit here (by calling uart_write).
}

// ...

const uart_config_t uart1 = {
  .tx = pin_init("TX", INPUT_PULLUP),
  .rx = pin_init("RX", INPUT),
  .baud_rate = 115200,
  .rx_data = on_uart_rx_data,
  .write_done = on_uart_write_done,
  .user_data = chip,
};