SWCU192 November 2021 CC1312R7 , CC1352P7 , CC2652P7 , CC2652R7
ICEMelter wakes up the JTAG power domain, that contains ICEPick and cJTAG modules and monitors the activities on the TCK-pin. When ICEMelter detects traffic on the TCK-pin (8 rising edges and 8 falling edges on TCK), it sends a power-up request to the AON WUC that powers up the JTAG power domain. There is a time-out built in the device that will reset the ICEMelter if the window from the third rising edge to the eighth rising edge exceeds 4 ms. The emulator must allow power-up time of at least 200 µs for JTAG power domain before sending remaining commands to JTAG interface.
TI recommends that care is taken to avoid unintentional traffic on the TCK-pin. This can for example happen if the TCK-pin is made accessible through a connector which is frequently connected and disconnected, or is located on a pin row that is touched during regular use. Unintentional traffic on the TCK-pin can cause the ICEMelter to power up the JTAG domain, which will add approximately 400 µA to any device mode (including Standby), and also set the Halt In Boot (HIB) flag. The HIB flag will halt the device on the subsequent boot (such as after any system reset other than pin reset or POR, or when entering Shutdown). Exiting Halt In Boot, clearing the HIB flag, and disabling the JTAG domain can only be done by a pin reset, a POR, or by using the JTAG interface itself.
The TCK pin has an internal pullup designed to avoid unintentional traffic due to noise, but it will not be sufficient if there is risk of external activity on the TCK-pin caused by unintentional touching or shorting of the pin. If it is not possible to guarantee that such activity does not happen, it is recommended that a strong external pullup is used, or even shorting the pin to the supply voltage through a zero-ohm resistor. The size of the pullup or whether the pin is shorted to the supply voltage, will be a tradeoff between robustness against unintentional external activity and the need for an accessible debug port. If the TCK pin is disabled by shorting it to the supply voltage, flash programming must be done using the bootloader.
It is possible to disable the input driver of the TCK pin by using [AON_IOC:TCKCTL]. Disabling the TCK pin impairs the debug sessions, so special care is required when disabling the TCK pin if the support for debugging the code is desired. TI strongly recommends to condition disabling of the TCK pin to when [FLASH:FWFLAG][2] is 1. [FLASH:FWFLAG][2] indicates whether or not HIB has taken place in the previous boot (0: HIB has taken place, 1: HIB has not taken place). Debugging a running target will not be possible if the TCK pin is disabled. However, debugging the device will be still possible if disabling the TCK pin is conditioned to when [FLASH:FWFLAG][2] is 1.