This project attempt to build a bicycle computer, suitable for 
collecting statistics about bike rides.

It aims at doing a much better job of measuring distance and speed 
compared to consumer grade bike computers.

First sensor is a standard IR barrier mounted to detect the passing
of the spokes on the rear wheel.

Chosen hardware platform is a standard STM32F103 microcontroller
on a blue-pill breakout board. See:
https://satoshinm.github.io/blog/171212_stm32_blue_pill_arm_development_board_first_look_bare_metal_programming.html

Primay function is to measure speed with high accuracy and log data.
Using  a photo sensor measuring time between spokes, we get about 30 reading per turn.

Available development support:
- In field programming. Ability to update firmware in field. Got serial update working,
  prepare hardware for it.
- Raw data logger. Collect measurements and dump on serial port for test/development.
  (Raw logger bandwidth: 12bytes x 200Hz x 10bits -> 24000 baud. Should be ok.)


Display: Drive small OLed. Typically controlled over i2c. Add i2c I/O expander for button
support. Plan for UI on separate board/box.

Theory of calculation:
- Create a number of small classes that are tied together. Each accepts some inputs and have a 
  'calc' function. Each also output some kind of state informing about validity of information.

Will need a number of 'clock-domains'.  Within a clock domain it should suffice with a 1 stage 
queue between modules. Between domains, there will be a need for buffering/interpolation.


Interrupt helpers:
- Set up each interrupt with a specific interrupt level.
- Define a class which list a number interrupts that can access it. It calculates
  a maximum priority level to use for protection.

- Create 'covers'. Name ISR that do the locking and generate code from that. Runtime will check
  that the correct ISR is running of fail. Cover and then restore mask given ISR priority.
- Wildcard covers will fail if incoming BASEPRI is to high. Othervise store current pri, protect
  and restore prev runtime value.

Hardware design:
Focus on a 2 part design:
- A frontend unit with OLED, buttons and the MCU.
- A Backend part with battery, sensor and interconnect to computers.

These is connected using a 4 wire cable between them. It carries
ground, power (+5V), sensor signal and serial TX.

The interconnect do:
- Supply 3 point conntection sensor.
- 4 point connector to front unit.
- USB connection to the battery.
- Connecter to computer serial port.
- Power switch.

Packaged into a case to be mounted close to the saddle on the bike


The frontend:
Connect MCU, buttons and OLed. (Future accelerometer?)
4-wire connector.
- Have ability for SW upgrade. Use Serial upgrade.
  Provide 6 pin if.

MCU pinout:
- A10 : usart1-RX.
- A9  : usart-TX.
- A2  : Pulse input.
- B7  : i2c Data display.
- B6  : i2c Clk display.
- B12 : Button 1.
- B13 : Button 2.