General-purpose I/Os (GPIO) RM0401
138/771 RM0401 Rev 3
6.3.2 I/O pin multiplexer and mapping
The microcontroller I/O pins are connected to onboard peripherals/modules through a
multiplexer that allows only one peripherals alternate function (AF) connected to an I/O pin
at a time. In this way, there can be no conflict between peripherals sharing the same I/O pin.
Each I/O pin has a multiplexer with sixteen alternate function inputs (AF0 to AF15) that can
be configured through the GPIOx_AFRL (for pin 0 to 7) and GPIOx_AFRH (for pin 8 to 15)
registers:
• After reset all I/Os are connected to the system’s alternate function 0 (AF0)
• The peripherals’ alternate functions are mapped from AF1 to AF13
• Cortex
®
-M4 with FPU EVENTOUT is mapped on AF15
This structure is shown in Figure 17 below.
In addition to this flexible I/O multiplexing architecture, each peripheral has alternate
functions mapped onto different I/O pins to optimize the number of peripherals available in
smaller packages.
To use an I/O in a given configuration, proceed as follows:
• System function
Connect the I/O to AF0 and configure it depending on the function used:
– JTAG/SWD, after each device reset these pins are assigned as dedicated pins
immediately usable by the debugger host (not controlled by the GPIO controller)
– RTC_REFIN: this pin should be configured in Input floating mode
– MCO1 and MCO2: these pins have to be configured in alternate function mode.
Note: You can disable some or all of the JTAG/SWD pins and so release the associated pins for
GPIO usage.
For more details please refer to Section 5.2.10: Clock-out capability.
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