Ole Andreas Utstumo
What makes you tick?
Within each individual muscle cell in our heart there is a difference of charge between the inside and the membrane. Ions start gushing out from the control node in the heart and into the resting cell, making this difference of charge decrease until the cell contracts. A moment later the ions flow back and the cell returns to it's resting state. This will spread like a wave through your heart, and is called a heartbeat!
While this wave of contracting cells is spreading throughout your heart, we can measure the difference of charge between the resting cells and the contracted cells as an electric potential, or voltage if you will. Amplifying this tiny signal and plotting this voltage on paper as it happens is basically the ECG -- the Electrocardiogram.
With the ECG you will be given a window into your heart so that serious abnormalities can be revealed. Patients can be examined with stationary equipment for a small period of time, and an analysis of the heart activity can indicate complications that should be thoroughly examined. However, abnormal heart activity in an otherwise perfectly normal heart might occur outside of the examination room.
The Heartbeat Logger is a portable device that that logs your ECG throughout the day and throughout the night, 24/7. While this certainly is nothing new, even as an open source project (see MobilECG), Heartbeat is a project that, aside being of personal value for me, is designed to be simple to use and understand, and might serve a purpose somewhere for someone.
Disclaimer: THIS IS BY NO MEANS A MEDICAL DEVICE. Use at your own risk.
Project index
Current status: Third revision arrived, awaiting time to continue.
Heartbeat Logger hardware and firmware files in GitHub
The heart and the ECG
Hardware
- System overview
- The analog front end
- The frontend's INA321 inamps
- The frontend's TSV634 opamps
- SAMD20 microcontroller
- Clock source
- Storage
- User interface
- Measuring the battery
- Voltage regulator
- Battery charger
Firmware
The oscillation loop maximal critical gain parameter (Gm_critmax) is indeed specified in the STM32L432KB datasheet:
Let's check out one of the crystals I went with in the KiCAD library:
So C_0 is between 2.0 and 0.6 pF. For the ESR:
We can set ESR = 35 k. We settle for a mid range load capacitance of 6 pF, which means our g_mcrit is calculated to be
Speaking of drive level...
There is a section in the guideline document detailing this. In short, the current flowing in and out of the crystal needs to be limited for safe operation. I might look at that when the circuit is constructed. I might also turn a blind eye to it, and assume it's going to work out well.
Skipping to section 3.3:
Which should mean that with a C_L= 6 pF and an assumed C_s = 5 pF, we're short of 1 pF, which we'll add by using two 2 pF caps in parallel (as seen by the crystal). We're not going to fine tune the crystal.
Hopping to section 3.8:
Further reading reveals that determining R_ADD involves adding resistance in series with the crystal until the oscillator is unable to start. Looking at the safety factor comparison table, it seems like LSE oscillators like this might be more stable in general... I feel I'm going to not bother unless there are any obvious issues ;-)
We're assuming the lowest drive level of the STM32 won't overdrive the crystal. We could check on that when it's all soldered up, but we're done here for now. 
The bottom ground layer is without any major traces, set aside for the pads and traces for the memory card reader and the BT module where I removed all unnecessary pads, in order to improve the EMC.
Mark Omo
jaromir.sukuba
David Scholten
It's so encouraging and satisfying to witness you seeing this project through. Thanks man.