diff --git a/machines/cortex-m/armv7em/stm32l432/lib/serial.c b/machines/cortex-m/armv7em/stm32l432/lib/serial.c
index ebf43a3b4..7470e96bf 100644
--- a/machines/cortex-m/armv7em/stm32l432/lib/serial.c
+++ b/machines/cortex-m/armv7em/stm32l432/lib/serial.c
@@ -2,6 +2,10 @@
 #include "serial.h"
 #include "tpl_app_define.h" /* buffer sizes */ 
 
+#ifndef SERIAL_TX_BUFFER_SIZE
+  #define SERIAL_TX_BUFFER_SIZE 16 
+#endif
+
 /* TX FIFO buffer
  * updated in interrupt handler
  */