I recall the there was a note in the manual that if debug is not used than the TRST must be pulled DOWN. It's not really a 'must', it's more of a recommendation. They just want to make sure the JTAG gets reset after POR. In version 3 of the hardware manual they went back and changed that note to make it more clear. Just remember that the JTAG unit also needs to drive that TRST line. One more note (that probably doesn't apply to you), when you boot a Linux kernel, if the TRST line is not high the system won't boot because of the way the Linux kernel initializes the break controllers in the Coresight.
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