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10 pin ARM Cortex JTAG

Posted: Thu Feb 15, 2018 8:30 pm
by JJ
Anyone know what to do with the KEY signal on a 10 pin connector? I've seen suggestions it can just be grounded, but I'm concerned this might short out a programmer. (I use a Flyswatter 2 with OpenOCD)

In the past I've used 20 and 14 pin JTAG but would like to reduce it to 10 to save board space (without going Tag-Connect) and fall in line with current recommendations.

This is for an LPC4078. Three options I can think of:

* Leave it floating
* Ground it
* Connect it to P2.10, the ISP enable pin (which currently has a weak 100k pullup on my board since I don't ever use the boot ROM).

I have a vague suspicion it's really intended for the third option... but if NOT and I connect it there it could make the board get stuck in the ISP on boot.

I can't seem to find ANY documentation discussing this...

Anyone know?
Will OpenOCD ever use it?
Is it electrically connected when adapted to a 20 pin programmer like the Flyswatter 2?

Re: 10 pin ARM Cortex JTAG

Posted: Thu Feb 15, 2018 8:33 pm
by JJ
Also, I'm concerned if it's an ISP enable it would be held high unless trying to enter ISP. If I ground it this would not be so great and might cause the programmer to shutdown on overload if it shorts (FT2232R I believe is what the FS2 uses, not sure if it's buffered).

Re: 10 pin ARM Cortex JTAG

Posted: Thu Feb 15, 2018 8:52 pm
by JJ
Looking at the schematic for the Olimex 20-to-10 adapter, they run this through an unpopulated resistor, so I guess I can just ground it. (That's the adapter I use.)

What about GNDDetect? Another pin to just ground?