LM3S317 Datasheet, PDF (247/551 Pages) List of Unclassifed Manufacturers – Microcontroller

Stellaris^® LM3S317 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.
Important: All GPIO pins are inputs by default (GPIODIR=0 and GPIOAFSEL=0), with the exception
of the five JTAG pins (PB7 and PC[3:0]). The JTAG pins default to their JTAG
functionality (GPIOAFSEL=1). A Power-On-Reset (POR) or asserting an external reset
(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 – If the JTAG pins are used as GPIOs in a design, PB7 and PC2 cannot have external pull-down
resistors connected to both of them at the same time. If both pins are pulled Low during reset, the
controller has unpredictable behavior. If this happens, remove one or both of the pull-down resistors,
and apply RST or power-cycle the part.
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
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.
July 14, 2014
247
Texas Instruments-Production Data