Most of the pins of the STM32 can be configured as input or output
and may be connected to either the GPIO ports or “alternative functions”
(other peripherals). As a standard naming convention, the pins are called by
their GPIO function – for example PA0 (bit 0 of port A) or PB9 (bit 9 of
port B). Indeed, the labeling of the discovery board follows this convention.
Subject to specific hardware constraints, each pin can be configured in the
modes illustrated in Figure 4.2.