diff --git a/Globals.h b/Globals.h
index a44e888..c082d60 100644
--- a/Globals.h
+++ b/Globals.h
@@ -97,7 +97,7 @@ const uint16_t RX_BLOCK_SIZE = 2U;
 const uint16_t TX_RINGBUFFER_SIZE = 500U;
 const uint16_t RX_RINGBUFFER_SIZE = 600U;
 
-#if defined(STM32F105xC)
+#if defined(STM32F105xC) || defined(__MK20DX256__)
 const uint16_t TX_BUFFER_LEN = 2000U;
 #else
 const uint16_t TX_BUFFER_LEN = 4000U;
diff --git a/SerialRB.h b/SerialRB.h
index a2ccb8b..ee244d2 100644
--- a/SerialRB.h
+++ b/SerialRB.h
@@ -32,7 +32,7 @@ Boston, MA  02110-1301, USA.
 #include <Arduino.h>
 #endif
 
-#if defined(STM32F105xC)
+#if defined(STM32F105xC) || defined(__MK20DX256__)
 const uint16_t SERIAL_RINGBUFFER_SIZE = 370U;
 #else
 const uint16_t SERIAL_RINGBUFFER_SIZE = 1000U;