nRF52840 JTAG disable and don't recover through any JTAG Debugger.

[Siddharth Kachhia]

Hi Team!

Currently I an working on disabling JTAG/Debug and SWD lines to protect code from read/write through any JTAG debugger.

For this, I have used nrfjprog utility and rbp command to disable JTAG read/write/erase. I have performed it and it works perfectly.
Now to enable JTAG again, I need to use nrfjprog --recover, which erase flash and UICR to enable flash rd/wr again. After --recover, I can do
flash img and rd/wr register through nrfjprog utility.

Question:
1) What does --recover do with nrfjprog utility ? Is it write/reset any ctrl-AP register to enable JTAG/Debug lines again ?
2) Can I disable --recover in board through any register so that in the field Flash read/write can be disable through any JTAG ?
3) If I want to disable JTAG/Debug lines in future, what are the options available for that ?

Thanks in Advance!

[Amanda Hsieh]

Hi, 

Now to enable JTAG again, I need to use nrfjprog --recover, which erase flash and UICR to enable flash rd/wr again. After --recover, I can do
flash img and rd/wr register through nrfjprog utility.

Yes, this is the new feature of APPROTECT in the nRF52840 Rev3. 

1) What does --recover do with nrfjprog utility ? Is it write/reset any ctrl-AP register to enable JTAG/Debug lines again ?

Writing image to disable ap protect, and Erasing user code and UICR flash areas. It erases the device and writes UICR.APPROTECT to HwDisabled (0x5A). 

2) Can I disable --recover in board through any register so that in the field Flash read/write can be disable through any JTAG ?

No, this is not support to disable --recover.

3) If I want to disable JTAG/Debug lines in future, what are the options available for that ?

To unlock the device debug port (for debugging etc.), execute the following steps to disable the access port protection:
1. Start with a CTRL-AP ERASEALL operation.
2. Program code compiled with MDK 8.40.2 or later, without ENABLE_APPROTECT defined.
3. Write HwDisabled (0x5A) to UICR.APPROTECT
4. Perform any reset to run the code. The programmed code from step 2 will open access port by writing to APPROTECT.DISABLE during startup.

See https://infocenter.nordicsemi.com/pdf/in_141_v1.1.pdf  

Also, have a look at this Working with the nRF52 Series' improved APPROTECT 

Regards,
Amanda

[Siddharth Kachhia]

Is it possible to fully disable JTAG and/or SWD?

[Amanda Hsieh]

No, not in software, but you could close the JTAG or SWD port in hardware. 

-Amanda

[Siddharth Kachhia]

How to erase/write Ctrl-AP port Reg from cmd line ?

[Siddharth Kachhia]

Ctrl-AP erase can also erase BPROT - Block Protection area of flash or not ?

If I have protected some flash area with BPROT then Ctrl-AP can erase it or not ?