LM3S2939 Datasheet, PDF (309/749 Pages) Texas Instruments – Stellaris® LM3S2939 Microcontroller

Stellaris^® LM3S2939 Microcontroller
Register 10: GPIO Alternate Function Select (GPIOAFSEL), offset 0x420
The GPIOAFSEL register is the mode control select register. Writing a 1 to any bit in this register
selects the hardware control for the corresponding GPIO line. All bits are cleared by a reset, therefore
no GPIO line is set to hardware control by default.
The GPIO commit control registers provide a layer of protection against accidental programming of
critical hardware peripherals. Protection is currently provided for the five JTAG/SWD pins (PB7 and
PC[3:0]). Writes to protected bits of the GPIO Alternate Function Select (GPIOAFSEL) register
(see page 309) are not committed to storage unless the GPIO Lock (GPIOLOCK) register (see
page 319) has been unlocked and the appropriate bits of the GPIO Commit (GPIOCR) register (see
page 320) have been set to 1.
Important: All GPIO pins are tri-stated by default (GPIOAFSEL=0, GPIODEN=0, GPIOPDR=0,
and GPIOPUR=0), with the exception of the five JTAG/SWD pins (PB7 and PC[3:0]).
The JTAG/SWD pins default to their JTAG/SWD functionality (GPIOAFSEL=1,
GPIODEN=1 and GPIOPUR=1). A Power-On-Reset (POR) or asserting RST puts both
groups of pins back to their default state.
While debugging systems where PB7 is being used as a GPIO, care must be taken to
ensure that a low value is not applied to the pin when the part is reset. Because PB7
reverts to the TRST function after reset, a Low value on the pin causes the JTAG
controller to be reset, resulting in a loss of JTAG communication.
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. This may lock the debugger out of the part. This
can be avoided with a software routine that restores JTAG functionality based on an external or software
trigger.
GPIO Alternate Function Select (GPIOAFSEL)
GPIO Port A base: 0x4000.4000
GPIO Port B base: 0x4000.5000
GPIO Port C base: 0x4000.6000
GPIO Port D base: 0x4000.7000
GPIO Port E base: 0x4002.4000
GPIO Port F base: 0x4002.5000
GPIO Port G base: 0x4002.6000
GPIO Port H base: 0x4002.7000
Offset 0x420
Type R/W, reset -
31
30
29
28
27
26
25
Type RO
RO
RO
RO
RO
RO
RO
Reset
0
0
0
0
0
0
0
15
14
13
12
11
10
9
reserved
Type RO
RO
RO
RO
RO
RO
RO
Reset
0
0
0
0
0
0
0
24
23
22
21
20
19
18
17
16
reserved
RO
RO
RO
RO
RO
RO
RO
RO
RO
0
0
0
0
0
0
0
0
0
8
7
6
5
4
3
2
1
0
AFSEL
RO
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
0
-
-
-
-
-
-
-
-
Bit/Field
31:8
Name
reserved
Type
RO
Reset
0x00
Description
Software should not rely on the value of a reserved bit. To provide
compatibility with future products, the value of a reserved bit should be
preserved across a read-modify-write operation.
June 18, 2012
309
Texas Instruments-Production Data