I now have a MI Band displaying Glucose without a paired iPhone. You just need the iPhone to push the calibration details to the device.
Lots to do here:
Lots to do here:
- Get the CC2500 code working on the Feather (SPI protocol)
- Get central to MI communication working. Adapt what people have done in Python, to work on the Arduino. See https://medium.com/machine-learning-world/how-i-hacked-xiaomi-miband-2-to-control-it-from-linux-a5bd2f36d3ad for a start.
- Also code the peripheral side so the feather can communicate with an iPhone, pushing the isig and retrieving the slope/intercept.
- Build the circuit
- Print a container
- Get a reasonable battery life
First started with everything on a breadboard, Feather NRF52 and a CC2500 board on a breakout.
Getting the CC2500 working wasn't too bad. Only some question as the where the Chip Select/Slave Select pin was on the feather. I recently found some mention that it was A8. My code has it on A3, since I couldn't find where it was mapped initially.
Central to MI took allot of time, knew that going in. To really understand what was going on, I had to get that working first in Python on a Raspberry PI. This guy had a great script: https://github.com/vshymanskyy/miband2-python-test .
Hardest part of the conversion is doing the AES 128 ECB encryption for the key exchange with the MI. This reminded me of communicating with the Dexcom back at the start of this blog where you had to calculate a CRC. In either case, there are lots of methods available, and lots of public implementations that may or may not work. I probably tried 10... In the end this did the trick.
Finally, very little out there for Central samples.
The peripheral side of things was tricky for me because of the way the iPhone caches BLE Characteristics. For a couple of days, I could enumerate my new characteristics from Linux, but the phone, and xCode and light blue wouldn't show anything. Turns out you need to restart your phone.
Modified my iPhone app to work with the new device and finally display the dexcom transmitter battery level.With that, squeezed together a circuit:
I may do a PCB in the future, I don't trust the wires. Also, would have been allot easier. The connections on the CC2500 are very small, not easy to solder to.
And printed another box, 2.25 x 1.6 x 0.4 (inches)
First week, could only get 12 hours on a charge. That works out to 9 ma/hr. Broke out the current measuring apparatus and started watching things. My initial central only sketch was hitting 9ma on pairing then settling down to 3ma, but the full code base was fluctuating between 9ma and 13ma after the first CC2500 receive event and refusing to drop below 9ma. Hmm.
Did some desperation googling, found a Nordic post on how some low level errors will prevent the Microcontroller from ever entering low power mode. A note here, you can run the NRF52 in debug mode and get some extra detail in the serial monitor. In this case, I noticed a few unhandled error messages, that don't crash the micro, but they prevent low power mode. An example of one, would be attempting to connect to a peripheral after you've already established a connection. The central examples that come with the feather are prone to this... Fixed these issues, battery life is now 3 ma/hr or lower. Codewise, doesn't appear that waitForEvent has any impact on battery, delay does.
99% of the time, the device is at 2.5ma, the CC2500 uses 27ma for 2 seconds every 5 minutes, and communication with the MI costs 9ma for a few seconds every 5 minutes or much much less, since I don't send value every 5 minutes unless the current glucose level is a concern.
Nothing to be gained space wise, so no point going to a smaller battery, done.
Display is a bit small, and the regular notifications are cleared very quickly. The real alerts vibrate until dismissed so you'll get to read those without issue. Like the VivoSmart, I'm not displaying glucose in the traditional manner. Example, if my blood sugar is in range, and not at risk of being high or low in the foreseeable future, nothing is sent to the band. If I'm really paranoid I can always check on the iPhone. Battery life on the MI is weeks. You can also still connect to MI Fit to get your step counts, although I don't let it pair.
See my github for the code.
