created SDdatalogger example

examples/SDdatalogger/README.md

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+This is a SDdatalogger, to log data from the accelerometer to the SD-card. It also functions as card reader, so you can easily get the data on your PC.
+
+To run, put the boot.py, cardreader.py and datalogger.py files on either the flash or the SD-card of your pyboard.
+Upon reset, the datalogger script is run and logs the data. If you press the user button after reset and hold it until the orange LED goes out, you enter the cardreader mode and the filesystem is mounted to your PC.

examples/SDdatalogger/README.md~

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+This is a SDdatalogger, to log data from the accelerometer to the SD-card. It also functions as card reader, so you can easily get the data on your PC.
+
+To run, put the boot.py, cardreader.py and datalogger.py files on either the flash or the SD-card of your pyboard.
+Upon reset, the datalogger script is run and logs the data. If you press the user button after reset and hold it until the orange LED goes out, you enter the cardreader mode and the filesystem is mounted to your PC.

examples/SDdatalogger/boot.py

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+# boot.py -- runs on boot-up
+# Let's you choose which script to run.
+# > To run 'datalogger.py':
+#       * press reset and do nothing else
+# > To run 'cardreader.py':
+#       * press reset
+#       * press user switch and hold until orange LED goes out
+
+import pyb
+
+pyb.LED(3).on()
+pyb.delay(2000)
+pyb.LED(4).on()
+pyb.LED(3).off()
+switch = pyb.Switch()                   # check if switch was pressed decision phase
+
+if switch():
+    pyb.usb_mode('CDC+MSC')
+    pyb.main('cardreader.py')           # if switch was pressed, run this
+else:
+    pyb.usb_mode('CDC+HID')
+    pyb.main('datalogger.py')           # if switch wasn't pressed, run this
+
+pyb.LED(4).off()

examples/SDdatalogger/cardreader.py

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+# cardread.py
+# This is called when the user enters cardreader mode. It does nothing.

examples/SDdatalogger/datalogger.py

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+# datalogger.py
+# Logs the data from the acceleromter to a file on the SD-card
+
+import pyb
+
+# creating objects
+accel = pyb.Accel()
+blue = pyb.LED(4)
+switch = pyb.Switch()
+
+# loop
+while True:
+
+    # start if switch is pressed
+    if switch():
+        pyb.delay(200)                      # delay avoids detection of multiple presses
+        blue.on()                           # blue LED indicates file open
+        log = open('1:/log.csv', 'w')       # open file on SD (SD: '1:/', flash: '0/)
+
+        # until switch is pressed again
+        while not switch():
+            t = pyb.millis()                            # get time
+            x, y, z = accel.filtered_xyz()              # get acceleration data
+            log.write('{},{},{},{}\n'.format(t,x,y,z))  # write data to file
+
+        # end after switch is pressed again
+        log.close()                         # close file
+        blue.off()                          # blue LED indicates file closed
+        pyb.delay(200)                      # delay avoids detection of multiple presses