TM4C1237H6PZ Datasheet, PDF (202/1307 Pages) Texas Instruments – Tiva Microcontroller

JTAG Interface
Because this sequence is a valid series of JTAG operations that could be issued, the ARM JTAG
TAP controller is not fully compliant to the IEEE Standard 1149.1. This instance is the only one
where the ARM JTAG TAP controller does not meet full compliance with the specification. Due to
the low probability of this sequence occurring during normal operation of the TAP controller, it should
not affect normal performance of the JTAG interface.
JTAG-to-SWD Switching
To switch the operating mode of the Debug Access Port (DAP) from JTAG to SWD mode, the
external debug hardware must send the switching preamble to the microcontroller. The 16-bit
TMS/SWDIO command for switching to SWD mode is defined as b1110.0111.1001.1110, transmitted
LSB first. This command can also be represented as 0xE79E when transmitted LSB first. The
complete switch sequence should consist of the following transactions on the TCK/SWCLK and
TMS/SWDIO signals:
1. Send at least 50 TCK/SWCLK cycles with TMS/SWDIO High to ensure that both JTAG and SWD
are in their reset states.
2. Send the 16-bit JTAG-to-SWD switch command, 0xE79E, on TMS/SWDIO.
3. Send at least 50 TCK/SWCLK cycles with TMS/SWDIO High to ensure that if SWJ-DP was already
in SWD mode before sending the switch sequence, the SWD goes into the line reset state.
To verify that the Debug Access Port (DAP) has switched to the Serial Wire Debug (SWD) operating
mode, perform a SWD READID operation. The ID value can be compared against the device's
known ID to verify the switch.
SWD-to-JTAG Switching
To switch the operating mode of the Debug Access Port (DAP) from SWD to JTAG mode, the
external debug hardware must send a switch command to the microcontroller. The 16-bit TMS/SWDIO
command for switching to JTAG mode is defined as b1110.0111.0011.1100, transmitted LSB first.
This command can also be represented as 0xE73C when transmitted LSB first. The complete switch
sequence should consist of the following transactions on the TCK/SWCLK and TMS/SWDIO signals:
1. Send at least 50 TCK/SWCLK cycles with TMS/SWDIO High to ensure that both JTAG and SWD
are in their reset states.
2. Send the 16-bit SWD-to-JTAG switch command, 0xE73C, on TMS/SWDIO.
3. Send at least 50 TCK/SWCLK cycles with TMS/SWDIO High to ensure that if SWJ-DP was already
in JTAG mode before sending the switch sequence, the JTAG goes into the Test Logic Reset
state.
To verify that the Debug Access Port (DAP) has switched to the JTAG operating mode, set the
JTAG Instruction Register (IR) to the IDCODE instruction and shift out the Data Register (DR). The
DR value can be compared against the device's known IDCODE to verify the switch.
4.4 Initialization and Configuration
After a Power-On-Reset or an external reset (RST), the JTAG pins are automatically configured for
JTAG communication. No user-defined initialization or configuration is needed. However, if the user
application changes these pins to their GPIO function, they must be configured back to their JTAG
functionality before JTAG communication can be restored. To return the pins to their JTAG functions,
enable the four JTAG pins (PC[3:0]) for their alternate function using the GPIOAFSEL register.
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June 12, 2014
Texas Instruments-Production Data