Part Number: TM4C1294NCPDT
Other Parts Discussed in Thread: EK-TM4C1294XL, UNIFLASH
Hello Everyone,
I am having trouble debugging a TM4C1294NCPDT on a custom board using CCS and an XDS110.
The strange part is that debugging works the first time. CCS connects, programs the device, and the code runs fine. But after that, I cannot debug again. Every attempt gives the error:
Unable to access the DAP, try reset the device or lower the clock
Power cycling the board does not help. I also tried lowering the JTAG clock in CCS, but the error remains.
Same Code is working perfectly on Launchpad : EK-TM4C1294XL
Initially I thought the device might be getting locked, so I followed the UniFlash/dbgjtag unlock procedure for TM4C12x devices (holding reset during power-up, dbgjtag unlock, etc.). The unlock completes successfully, but debugging is still not possible afterward.
It feels like something in the firmware is breaking debug access after the first run (possibly clock/PLL configuration or reset behavior), but I’m not sure what exactly causes CCS to lose access to the DAP.
Has anyone seen this behavior on TM4C1294 before?
Is it possible for early clock configuration or reset handling to block DAP access until the flash is erased?
Any recommendations on what to check on the hardware or firmware side would be helpful.
I am also attaching Schematic of Custom PCB. Please have a look.
