Nike+ is a clever little system designed by Apple and Nike to infer the runner's speed and augment the running experience. The runner places a small chip in his shoe which transmits data to the iPod using a proprietary RF-based protocol. The chip contains a piezoelectric cell which measures how long the foot exerted pressure on the ground. According to Apple's FAQ, this contact time is directly related to your pace. In this post I provide a snippet of python code for collecting data through Sparkfun's adapter.

Sparkfun dissected the transmitter and receiver and currently sell a Nike+ Serial to USB adapter. There's been a number of notable projects since, and quite a lot of interest in the system for general hackery.

I wanted to use Nike+ for my own project (a running bib that would automatically display the runner's speed on the back). Unfortunately, the only available implementations were in Visual Basic and perl, neither of which work on Mac. Here's a small script for Python on Mac OS X to collect Nike+ data using SparkFun's adapter.

#!/usr/bin/env python
import serial
from hexbyte import *

def readbytes(number):
    buf = ''
    for i in range(number):
        byte = ser.read()
        buf += byte

    return buf

# open the appropriate serial port
ser = serial.Serial('/dev/tty.usbserial-A6007uDh', 57600, bytesize=serial.EIGHTBITS)

# send the following init string to the Nike+ device:
init1 = 'FF 55 04 09 07 00 25 C7'
ser.write(HexToByte(init1))

# listen for the response string: FF 55 04 09 00 00 07 EC
response1 = ByteToHex(readbytes(8))
assert response1 == 'FF 55 04 09 00 00 07 EC'

# send the second init string
init2 = 'FF 55 02 09 05 F0'
ser.write(HexToByte(init2))

# listen for the response string: FF 55 04 09 06 00 25 C8
response2 = ByteToHex(readbytes(8))
assert response2 == 'FF 55 04 09 06 00 25 C8'

# now we're ready to listen for actual data
print "nike+ initialized. listening for data"

while True:
    byte = ser.read()
    # if a byte is coming down the port,
    if byte:
        # get the rest of the message (34 chars)
        message = byte + readbytes(33)
        # and decipher it
        data = {
            'number': ByteToHex(message[11]),
            'uid': ByteToHex(message[7:11]),
            'data': ByteToHex(message[12:]),
        }
        print data

You'll also need hexbyte.py, which contains convenience conversion methods between binary and hex.

I'm still unable to fully make sense of this data. Firstly, each step seems to inexplicably generate 8 packets instead of one. Second, there are 22 bytes in the Nike+ data with an unknown structure, probably containing pressure duration data. If someone figures out how to make sense of this please let me know!

Update (June 2013): Dmitry Grinberg has published a much more thorough reverse engineering blog post.