diff --git a/drivers/avr/usart1.c b/drivers/avr/usart1.c
index 4e4349a..e28fae7 100644
--- a/drivers/avr/usart1.c
+++ b/drivers/avr/usart1.c
@@ -54,7 +54,8 @@ usart1_write_ready_p (void)
 }
 
 
-/** Return non-zero if transmitter finished.  */
+/** Return non-zero if transmission finished.  This is useful for
+    half-duplex operation.  */
 bool
 usart1_write_finished_p (void)
 {
diff --git a/drivers/avr/usart1.h b/drivers/avr/usart1.h
index d0bf26d..af9b069 100644
--- a/drivers/avr/usart1.h
+++ b/drivers/avr/usart1.h
@@ -37,7 +37,8 @@ bool
 usart1_write_ready_p (void);
 
 
-/** Return non-zero if transmitter finished.  */
+/** Return non-zero if transmission finished.  This is useful for
+    half-duplex operation.  */
 bool
 usart1_write_finished_p (void);