this information to be shifted out on TDOduring the TAP controller’s SHIFT states. While the sampled
data is being shifted out of the chain on TDO, new data is being shifted into the serial shift register
on TDI. This new data is stored in the parallel load register during the TAP controller’s UPDATE
states. Each of the shift registers is discussed in detail in “Register Descriptions” on page 171.
4.3.4 Operational Considerations
Certain operational parameters must be considered when using the JTAG module. Because the
JTAG pins can be programmed to be GPIOs, board configuration and reset conditions on these
pins must be considered. In addition, because the JTAG module has integrated ARM Serial Wire
Debug, the method for switching between these two operational modes is described below.
4.3.4.1 GPIO Functionality
When the microcontroller is reset with either a POR or RST, the JTAG/SWD port pins default to their
JTAG/SWD configurations. The default configuration includes enabling digital functionality (DEN[3:0]
set in the Port C GPIO Digital Enable (GPIODEN) register), enabling the pull-up resistors (PUE[3:0]
set in the Port C GPIO Pull-Up Select (GPIOPUR) register), disabling the pull-down resistors
(PDE[3:0] cleared in the Port C GPIO Pull-Down Select (GPIOPDR) register) and enabling the
alternate hardware function (AFSEL[3:0] set in the Port C GPIO Alternate Function Select
(GPIOAFSEL) register) on the JTAG/SWD pins. See page 430, page 436, page 438, and page 441.
It is possible for software to configure these pins as GPIOs after reset by clearing AFSEL[3:0] in
the Port C GPIOAFSEL register. If the user does not require the JTAG/SWD port for debugging or
board-level testing, this provides four more GPIOs for use in the design.
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 ash 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 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 430), GPIO Pull Up Select (GPIOPUR) register (see page 436), GPIO Pull-Down
Select (GPIOPDR) register (see page 438), and GPIO Digital Enable (GPIODEN) register (see
page 441) are not committed to storage unless the GPIO Lock (GPIOLOCK) register (see page 443)
has been unlocked and the appropriate bits of the GPIO Commit (GPIOCR) register (see page 444)
have been set.
4.3.4.2 Communication with JTAG/SWD
Because the debug clock and the system clock can be running at different frequencies, care must
be taken to maintain reliable communication with the JTAG/SWD interface. In the Capture-DR state,
the result of the previous transaction, if any, is returned, together with a 3-bit ACK response. Software
should check the ACK response to see if the previous operation has completed before initiating a
new transaction. Alternatively, if the system clock is at least 8 times faster than the debug clock
(TCK or SWCLK), the previous operation has enough time to complete and the ACK bits do not have
to be checked.
July 24, 2012168
Texas Instruments-Production Data
JTAG Interface
OBSOLETE: TI has discontinued production of this device.
