diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
index 130dcf9d36..234b75a1b1 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
@@ -1093,6 +1093,7 @@ void UARTDriver::write_pending_bytes(void)
     if (_flow_control == FLOW_CONTROL_AUTO) {
         if (_first_write_started_us == 0) {
             _first_write_started_us = AP_HAL::micros();
+            _total_written = 0;
         }
 #ifndef HAL_UART_NODMA
         if (tx_dma_enabled) {