Development Notes

All reasonable attempts were made to keep the Grbl 1.1f source intact. Whenever possible, calls to STM32 hardware are contained in:
```
#ifdef STM32
...
#endif
```
For reference, the older AVR codes are kept in:
```
#ifdef ATMEGA328P
...
#endif
```
ST TrueStudio (Atollic) IDE, with CubeMX for pins/timers/peripherals selection.
Common /grbl and /stm32 folders for three sub-branches under /Atollic/:
- F13 : 3-Axis running STM32F103
- F16 : 6-Axis running STM32F103. Conditional compilation symbols available for compiling 3, 4, 5, 6 axis.
- F46 : 6-Axis running STM32F407. Conditional compilation symbols available for compiling 3, 4, 5, 6 axis.

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