diff --git a/libraries/AP_HAL_ChibiOS/RCOutput.cpp b/libraries/AP_HAL_ChibiOS/RCOutput.cpp
index adbef52761..74b38e9f49 100644
--- a/libraries/AP_HAL_ChibiOS/RCOutput.cpp
+++ b/libraries/AP_HAL_ChibiOS/RCOutput.cpp
@@ -1727,7 +1727,7 @@ bool RCOutput::serial_write_bytes(const uint8_t *bytes, uint16_t len)
  */
 void RCOutput::serial_bit_irq(void)
 {
-    uint32_t now = AP_HAL::micros();
+    uint16_t now = AP_HAL::micros16();
     uint8_t bit = palReadLine(irq.line);
     bool send_signal = false;
 
@@ -1746,7 +1746,7 @@ void RCOutput::serial_bit_irq(void)
             irq.bitmask = 0;
         }
     } else {
-        systime_t dt = now - irq.byte_start_tick;
+        uint16_t dt = now - irq.byte_start_tick;
         uint8_t bitnum = (dt+(irq.bit_time_tick/2)) / irq.bit_time_tick;
 
         if (bitnum > 10) {
diff --git a/libraries/AP_HAL_ChibiOS/RCOutput.h b/libraries/AP_HAL_ChibiOS/RCOutput.h
index 2a56b90623..4df5473ef3 100644
--- a/libraries/AP_HAL_ChibiOS/RCOutput.h
+++ b/libraries/AP_HAL_ChibiOS/RCOutput.h
@@ -332,7 +332,7 @@ private:
         // serial output
         struct {
             // expected time per bit
-            uint32_t bit_time_us;
+            uint16_t bit_time_us;
 
             // channel to output to within group (0 to 3)
             uint8_t chan;
@@ -411,7 +411,7 @@ private:
         ioline_t line;
 
         // time the current byte started
-        uint32_t byte_start_tick;
+        uint16_t byte_start_tick;
 
         // number of bits we have read in this byte
         uint8_t nbits;
@@ -423,7 +423,7 @@ private:
         uint16_t byteval;
 
         // expected time per bit in micros
-        uint32_t bit_time_tick;
+        uint16_t bit_time_tick;
 
         // the bit value of the last bit received
         uint8_t last_bit;