Programatically set debug mode

[Robin]

Is there any way to programatically set the 51822 into debug mode following a power on reset, or disable pin reset in normal mode? 

Here is a synopsis of the issue:

Board design info:
1.) Intention was to use standard uUSB connector for both charging and programming interface.  Seems like a perfectly reasonable expectation, no?
2.) Board implemented to do so.  Uses USB signal pins for SWDIO & SWCLK.and PWR pins for charging.
Recent Findings:
1.) Off-the-shelf USB charges generally ground the signal pins, causing 51822 to be held in reset when chip is in normal mode.
2.) Programming leaves chip in debug mode with nReset disabled. Until a power loss there was no evidence of this issue.
3.) Once battery fully discharges, or board is power reset, chip is returned to normal mode when powered up, DIO pin becomes active nRESET.
4.) No known way to either a.) disable reset pin in normal mode, b.) reassign nRESET pin, or c.) programmatically set chip to debug mode.
5.) Reset register on/off only works in debug mode.
Current status:
1.) Presumably enable of POFCON will protect code/data on battery crashes.
2.) Prospective temporary work around (unacceptable for production) is to use data USB Isolators when charging.  Limited testing show this to be ~90% effective at preventing a hard reset when attaching a charger.  When it does cause a hard reset, the chip appears to reboots properly.
Long term fix, request for help:
1.). Seriously hoping Nordic can offer a way to either disable nRESET in normal mode, or reassign nRESETto different pin.  Otherwise the board will need to be redesigned to segregate charging and programming.

2.) Yet another costly board revision.

HELP PLEASE.

[ketiljo]

The reset pin can't be assigned to another pin, sorry. Neither can it be disabled in normal mode. 

It's generally not a good idea to se a widely used connector like USB for something else so the best here will be to either use cables with only VBUS and GND and no data lines or re-design the board to have the programming pins on a separate connector. There's always a risk that someone uses a normal USB cable here so the latter is really the safest way,.

[Robin]

Hello Ketiljo,

Thank you for the response..

Please answer the following question relative to chip capabilities, not Nordic policy:

Would it be physically possible to disable the nRESET pin with something along the lines of a "custom soft device" implementation?

Thank you,

Robin @ TL 

[ketiljo]

The reset pin is shared with the debug interface (SWDIO) on the nRF51-series devices. There's a HW block that enables the debugger/programmer after reset has been toggled a number of times. This is described in the reference manual, chapter 11

https://infocenter.nordicsemi.com/pdf/nRF51_RM_v3.0.1.pdf

There are no registers that can override this, sorry.

[Robin]

Hello again Ketiljo, and thank you for your response,

So, if I am understanding you correctly, it would indicate that the HW block is hard (mask) programmed.  Is this essentially correct?  Just a curiosity on the core technology of the chip.

Robin @ TL 

[ketiljo]

There are a lot of digital circuitry around the MCU that ties the different parts together. NAND gates are heavily used to perform logical operations. Most of this is done by design and not really programmed. The only way to change this it to change the metal routing layers that connects the gates together, just like a good old fashioned TTL circuits on a PCB.