LM4F110H5QR Datasheet, PDF (642/1155 Pages) Texas Instruments – Microcontroller

General-Purpose Input/Outputs (GPIOs)
Register 10: GPIO Alternate Function Select (GPIOAFSEL), offset 0x420
The GPIOAFSEL register is the mode control select register. If a bit is clear, the pin is used as a
GPIO and is controlled by the GPIO registers. Setting a bit in this register configures the
corresponding GPIO line to be controlled by an associated peripheral. Several possible peripheral
functions are multiplexed on each GPIO. The GPIO Port Control (GPIOPCTL) register is used to
select one of the possible functions. Table 20-5 on page 1084 details which functions are muxed on
each GPIO pin. The reset value for this register is 0x0000.0000 for GPIO ports that are not listed
in the table below.
Important: All GPIO pins are configured as GPIOs and tri-stated by default (GPIOAFSEL=0,
GPIODEN=0, GPIOPDR=0, GPIOPUR=0, and GPIOPCTL=0), with the exception of
the pins shown in the table below. A Power-On-Reset (POR) or asserting RST puts the
pins back to their default state.
Table 10-7. GPIO Pins With Non-Zero Reset Values
GPIO Pins
PA[1:0]
PA[5:2]
PB[3:2]
PC[3:0]
Default State
UART0
SSI0
I²C0
JTAG/SWD
GPIOAFSEL GPIODEN GPIOPDR GPIOPUR
0
0
0
0
0
0
0
0
0
0
0
0
1
1
0
1
GPIOPCTL
0x1
0x2
0x3
0x1
The GPIO commit control registers provide a layer of protection against accidental
programming of critical hardware signals including the GPIO pins that can function as
JTAG/SWD signals and the NMI signal. The commit control process must be followed
for these pins, even if they are programmed as alternate functions other than JTAG/SWD
or NMI, see “Commit Control” on page 628.
Caution – It is possible to create a software sequence that prevents the debugger from connecting to
the Stellaris microcontroller. If the program code loaded into flash immediately changes the JTAG pins
to their GPIO functionality, the debugger may not have enough time to connect and halt the controller
before the JTAG pin functionality switches. As a result, the debugger may be locked out of the part.
This issue can be avoided with a software routine that restores JTAG functionality based on an external
or software trigger.
The GPIO commit control registers provide a layer of protection against accidental programming of
critical hardware peripherals. Protection is provided for the GPIO pins that can be used as the four
JTAG/SWD pins (PC[3:0])and the NMI pin (PD7 and PF0). Writes to protected bits of the GPIO
Alternate Function Select (GPIOAFSEL) register (see page 642), GPIO Pull Up Select (GPIOPUR)
register (see page 648), GPIO Pull-Down Select (GPIOPDR) register (see page 650), and GPIO
Digital Enable (GPIODEN) register (see page 653) are not committed to storage unless the GPIO
Lock (GPIOLOCK) register (see page 655) has been unlocked and the appropriate bits of the GPIO
Commit (GPIOCR) register (see page 656) have been set.
When using the I²C module, in addition to setting the GPIOAFSEL register bits for the I²C clock
and data pins, the pins should be set to open drain using the GPIO Open Drain Select (GPIOODR)
register (see examples in “Initialization and Configuration” on page 628).
642
April 25, 2012
Texas Instruments-Advance Information