diff --git a/cores/esp8266/Arduino.h b/cores/esp8266/Arduino.h
index 6db13a088..9cc4ad08b 100644
--- a/cores/esp8266/Arduino.h
+++ b/cores/esp8266/Arduino.h
@@ -229,6 +229,9 @@ long random(long, long);
 void randomSeed(unsigned int);
 long map(long, long, long, long, long);
 
+// Debugging functions
+void hexdump(uint8_t *mem, uint32_t len, uint8_t cols = 16);
+
 #endif
 
 #include "pins_arduino.h"
diff --git a/cores/esp8266/debug.cpp b/cores/esp8266/debug.cpp
new file mode 100644
index 000000000..c42016b04
--- /dev/null
+++ b/cores/esp8266/debug.cpp
@@ -0,0 +1,21 @@
+/*
+ * debug.c
+ *
+ *  Created on: 13.05.2015
+ *      Author: Markus Sattler
+ */
+
+#include "Arduino.h"
+
+void ICACHE_RAM_ATTR hexdump(uint8_t *mem, uint32_t len, uint8_t cols) {
+    os_printf("\n[HEXDUMP] Address: 0x%08X len: 0x%X (%d)", mem, len, len);
+    for(uint32_t i = 0; i < len; i++) {
+        if(i % cols == 0) {
+            os_printf("\n[0x%08X] 0x%08X: ", mem, i);
+        }
+        os_printf("%02X ", *mem);
+        mem++;
+    }
+    os_printf("\n");
+}
+
diff --git a/libraries/SPI/SPI.cpp b/libraries/SPI/SPI.cpp
index be4627740..c96c4fcb8 100644
--- a/libraries/SPI/SPI.cpp
+++ b/libraries/SPI/SPI.cpp
@@ -308,6 +308,13 @@ void SPIClass::write32(uint32_t data, bool msb) {
     while(SPI1CMD & SPIBUSY) {}
 }
 
+/**
+ * Note:
+ *  data need to be aligned to 32Bit
+ *  or you get an Fatal exception (9)
+ * @param data uint8_t *
+ * @param size uint32_t
+ */
 void SPIClass::writeBytes(uint8_t * data, uint32_t size) {
     while(size) {
         if(size > 64) {
@@ -340,6 +347,15 @@ void SPIClass::writeBytes_(uint8_t * data, uint8_t size) {
     while(SPI1CMD & SPIBUSY) {}
 }
 
+
+/**
+ * Note:
+ *  data need to be aligned to 32Bit
+ *  or you get an Fatal exception (9)
+ * @param data uint8_t *
+ * @param size uint8_t  max for size is 64Byte
+ * @param repeat uint32_t
+ */
 void SPIClass::writePattern(uint8_t * data, uint8_t size, uint32_t repeat) {
     if(size > 64) return; //max Hardware FIFO
 
@@ -376,6 +392,14 @@ void SPIClass::writePattern_(uint8_t * data, uint8_t size, uint8_t repeat) {
     writeBytes(&buffer[0], bytes);
 }
 
+/**
+ * Note:
+ *  in and out need to be aligned to 32Bit
+ *  or you get an Fatal exception (9)
+ * @param out uint8_t *
+ * @param in  uint8_t *
+ * @param size uint32_t
+ */
 void SPIClass::transferBytes(uint8_t * out, uint8_t * in, uint32_t size) {
     while(size) {
         if(size > 64) {