diff --git a/atmel-samd/main.c b/atmel-samd/main.c
index 555a91e12c..9c9f59d160 100644
--- a/atmel-samd/main.c
+++ b/atmel-samd/main.c
@@ -271,8 +271,14 @@ void MP_WEAK __assert_func(const char *file, int line, const char *func, const c
 struct usart_module usart_instance;
 
 #ifdef ENABLE_MICRO_TRACE_BUFFER
-#define TRACE_BUFFER_SIZE 256
-__attribute__((__aligned__(TRACE_BUFFER_SIZE * sizeof(uint32_t)))) uint32_t mtb[TRACE_BUFFER_SIZE];
+// Stores 2 ^ TRACE_BUFFER_MAGNITUDE_PACKETS packets.
+// 7 -> 128 packets
+#define TRACE_BUFFER_MAGNITUDE_PACKETS 7
+// Size in uint32_t. Two per packet.
+#define TRACE_BUFFER_SIZE (1 << (TRACE_BUFFER_MAGNITUDE_PACKETS + 1))
+// Size in bytes. 4 bytes per uint32_t.
+#define TRACE_BUFFER_SIZE_BYTES (TRACE_BUFFER_SIZE << 2)
+__attribute__((__aligned__(TRACE_BUFFER_SIZE_BYTES))) uint32_t mtb[TRACE_BUFFER_SIZE];
 #endif
 
 // Serial number as hex characters.
@@ -292,11 +298,9 @@ void load_serial_number(void) {
 
 void samd21_init(void) {
 #ifdef ENABLE_MICRO_TRACE_BUFFER
-    memset(mtb, 0, sizeof(mtb));
-    // AWIDTH is 15.
     REG_MTB_POSITION = ((uint32_t) (mtb - REG_MTB_BASE)) & 0xFFFFFFF8;
-    REG_MTB_FLOW = ((uint32_t) mtb + TRACE_BUFFER_SIZE * sizeof(uint32_t)) & 0xFFFFFFF8;
-    REG_MTB_MASTER = 0x80000000 + 6;
+    REG_MTB_FLOW = (((uint32_t) mtb - REG_MTB_BASE) + TRACE_BUFFER_SIZE_BYTES) & 0xFFFFFFF8;
+    REG_MTB_MASTER = 0x80000000 + (TRACE_BUFFER_MAGNITUDE_PACKETS - 1);
 #endif
 
     load_serial_number();