LM3S9B81 Datasheet, PDF (318/1155 Pages) Texas Instruments – Stellaris® LM3S9B81 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 23-5 on page 1043 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 9-8. 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
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
1
GPIOPCTL
0x1
0x1
0x1
0x3
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 currently provided for the NMI pin (PB7) and the four
JTAG/SWD pins (PC[3:0]). Writes to protected bits of the GPIO Alternate Function Select
(GPIOAFSEL) register (see page 318), GPIO Pull Up Select (GPIOPUR) register (see page 324),
GPIO Pull-Down Select (GPIOPDR) register (see page 326), and GPIO Digital Enable (GPIODEN)
register (see page 329) are not committed to storage unless the GPIO Lock (GPIOLOCK) register
(see page 331) has been unlocked and the appropriate bits of the GPIO Commit (GPIOCR) register
(see page 332) 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 303).
318
June 29, 2010
Texas Instruments-Advance Information