This chapter describes all functional blocks (units) implemented in GEX at the time of publication of this work. Each unit supports a different set of binary commands and events. The term "unit" will be used to refer to both unit types (drivers) or their instances, where the distinction is not important.
Unit types are named in uppercase (e.g. SPI, 1WIRE, NPX) in the INI file and in the list of units. Unit instances can be named in any way the user desires; Using lowercase makes it easier to distinguish them from unit types. It is advisable to use descriptive names, e.g. not "pin1" but rather "button".
Several units facilitate an access to a group of GPIO pins, such as the digital input and output units, or the SPI unit's slave select pins. The STM32 microcontroller's ports have 16 pins each, most of which can be configured to one of several alternate functions (e.g. SPI, PWM outputs, ADC input). As a consequence, it's common to be left with a discontiguous group of pins after assigning all the alternate functions needed by an application.
For instance, we could only have the pins 0, 1, 12--15 available on a GPIO port. GEX provides a helpful abstraction to bridge the gaps in the port: The selected pins are packed together and represented, in commands and events, as a block of six pins (0x3F) instead of their original positions in the register (0xF003). This scheme is shown in figure \ref{fig:pin-packing}. The translation is done in the unit driver and works transparently, as if the block of pins had no gaps---all the referenced pins are updated simultaneously without glitches. Where pin numbers are used, the order in the packed word should be provided---in our example, that would be 0--5, counting from the least significant bit.
