diff --git a/firmware/inc/data.h b/firmware/inc/data.h
index 48cde6a..502eeb4 100644
--- a/firmware/inc/data.h
+++ b/firmware/inc/data.h
@@ -53,6 +53,9 @@ typedef struct tracker_datapoint {
   float radio_die_temperature;  /* ºC */
   uint32_t xosc_error;          /* Hertz */
 
+  /* system */
+  uint8_t flash_status;
+
 } tracker_datapoint;
 
 
diff --git a/firmware/inc/flash.h b/firmware/inc/flash.h
new file mode 100644
index 0000000..94cf407
--- /dev/null
+++ b/firmware/inc/flash.h
@@ -0,0 +1,35 @@
+/*
+ * Function related to the flash memory
+ * Copyright (C) 2016  Richard Meadows <richardeoin>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#ifndef FLASH_H
+#define FLASH_H
+
+enum flash_state {
+  FLASH_GOOD,                   /* checksum matches */
+  FLASH_BAD_CSUM,               /* mismatch */
+};
+
+enum flash_state check_flash_state(void);
+
+#endif /* FLASH_H */
diff --git a/firmware/inc/memory.h b/firmware/inc/memory.h
index d6f5e78..9de0fa1 100644
--- a/firmware/inc/memory.h
+++ b/firmware/inc/memory.h
@@ -50,6 +50,7 @@
 
 void mem_chip_erase(void);
 void mem_read_memory(uint32_t address, uint8_t* buffer, uint32_t length);
+void mem_write_word(uint32_t address, uint32_t word);
 void mem_write_page(uint32_t address, uint8_t* buffer, uint16_t length);
 void mem_erase_sector(uint32_t address);
 
diff --git a/firmware/src/flash.c b/firmware/src/flash.c
new file mode 100644
index 0000000..b03f647
--- /dev/null
+++ b/firmware/src/flash.c
@@ -0,0 +1,116 @@
+/*
+ * Function related to the flash memory
+ * Copyright (C) 2016  Richard Meadows <richardeoin>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#include <stdlib.h>
+
+#include "samd20.h"
+#include "memory.h"
+#include "flash.h"
+
+/**
+ * we put our checksum at the last address in flash
+ */
+volatile unsigned int* flash_checksum =
+  (unsigned int*)(FLASH_ADDR + FLASH_SIZE - 4);
+
+
+// ---------------------------- crc32cx --------------------------------
+
+/* This is crc32b modified to load the message a fullword at a time.
+   It assumes the message is word aligned and consists of an integral
+   number of words before the 0-byte that marks the end of the message.
+   This works only on a little-endian machine.
+   Not counting the table setup (which would probably be a separate
+   function), this function should be doable in 3 + 22w instructions, where
+   w is the number of fullwords in the input message. This is equivalent to
+   3 + 5.5n instructions, where n is the number of bytes. 1.25 of those 5.5
+   instructions are loads.
+   This is Exercise 1 in the text. C.f. Christopher Dannemiller,
+   who got it from Linux Source base,
+   www.gelato.unsw.edu.au/lxr/source/lib/crc32.c, lines 105-111. */
+
+unsigned int crc32cx(unsigned int *ptr, size_t length)
+{
+  int j;
+  unsigned int byte, crc, mask;
+  unsigned int table[256];
+
+  length &= ~0x3;               /* must be mutiple of 4 */
+
+  /* Set up the table */
+  for (byte = 0; byte <= 255; byte++) {
+    crc = byte;
+    for (j = 7; j >= 0; j--) {    // Do eight times.
+      mask = -(crc & 1);
+      crc = (crc >> 1) ^ (0xEDB88320 & mask);
+    }
+    table[byte] = crc;
+  }
+
+  /* Through with table setup, now calculate the CRC. */
+  crc = 0xFFFFFFFF;
+  while (length != 0) {
+    crc = crc ^ *ptr;
+    crc = (crc >> 8) ^ table[crc & 0xFF];
+    crc = (crc >> 8) ^ table[crc & 0xFF];
+    crc = (crc >> 8) ^ table[crc & 0xFF];
+    crc = (crc >> 8) ^ table[crc & 0xFF];
+
+    ptr++; length -= 4;
+  }
+  return ~crc;
+}
+
+/**
+ * 32 bit checksum for the whole memory space
+ */
+unsigned int checksum_memory(void)
+{
+  /* do crc  */
+  return crc32cx((unsigned int)FLASH_ADDR, /* start */
+                 FLASH_SIZE - 4);          /* length */
+}
+
+/**
+ * checks if memory checksum is good
+ */
+enum flash_state check_flash_state(void)
+{
+  unsigned int calculated = checksum_memory();
+
+  if (*flash_checksum == 0xFFFFFFFF) { /* not written */
+    /* write it */
+    mem_write_word((uint32_t)flash_checksum, calculated);
+
+    return FLASH_GOOD;
+
+  } else {                      /* written */
+    /* check it */
+    if (calculated == *flash_checksum) {
+      return FLASH_GOOD;
+    } else {
+      return FLASH_BAD_CSUM;
+    }
+  }
+}
diff --git a/firmware/src/memory.c b/firmware/src/memory.c
index 713e717..fabfd33 100644
--- a/firmware/src/memory.c
+++ b/firmware/src/memory.c
@@ -91,6 +91,34 @@ void mem_read_memory(uint32_t address, uint8_t* buffer, uint32_t length)
 {
   memcpy(buffer, nvm_section + address, length);
 }
+/**
+ * Write single word
+ */
+void mem_write_word(uint32_t address, uint32_t word)
+{
+#ifdef FIX_ERRATA_REV_C_FLASH_10804
+  /* save CTRLB and disable cache */
+  uint32_t temp = NVMCTRL->CTRLB.reg;
+  NVMCTRL->CTRLB.reg |= NVMCTRL_CTRLB_CACHEDIS;
+#endif
+
+  /* write address */
+  NVMCTRL->ADDR.reg = (uint32_t)(address) >> 1;
+  /* write data. length must be multiple of two */
+  *(uint32_t*)address = word;
+  /* unlock */
+  NVMCTRL->CTRLA.reg = NVMCTRL_CTRLA_CMDEX_KEY | NVMCTRL_CTRLA_CMD_UR;
+  /* write page */
+  NVMCTRL->CTRLA.reg = NVMCTRL_CTRLA_CMDEX_KEY | NVMCTRL_CTRLA_CMD_WP;
+  _mem_wait_for_done();
+  /* lock */
+  NVMCTRL->CTRLA.reg = NVMCTRL_CTRLA_CMDEX_KEY | NVMCTRL_CTRLA_CMD_LR;
+
+#ifdef FIX_ERRATA_REV_C_FLASH_10804
+  /* restore CTRLB */
+  NVMCTRL->CTRLB.reg = temp;
+#endif
+}
 /**
  * Write 64-byte page. Address should be page aligned
  */
diff --git a/firmware/src/sequencer.c b/firmware/src/sequencer.c
index f80d81f..f5ca688 100644
--- a/firmware/src/sequencer.c
+++ b/firmware/src/sequencer.c
@@ -35,6 +35,7 @@
 #include "accumulator.h"
 #include "battery.h"
 #include "rtc.h"
+#include "flash.h"
 
 
 void rtty_telemetry(struct tracker_datapoint* dp);
@@ -162,4 +163,9 @@ void run_sequencer(uint32_t n, uint32_t cycle_time_s)
 
   /* Battery */
   update_battery(dp);
+
+  /* Flash */
+  if ((n % 100) == 1) {
+    dp->flash_status = check_flash_state();
+  }
 }