I'm developing an application on a very low pin count uController (LPC-810M, 8 pins). So, it's necessary to use the same pin for both GPIO and SWCLK/SWDIO.

When debugging, there's no issue since there will be only 1 driver on the bus.

When not debugging and the pin is outputting a GPIO signal, there's the risk of short circuiting.

Can this be safely done?

I saw this infocenter.arm.com/help/index.…oc.dui0499b/CHDHDGGG.html, which says there's some current limiting protection inside the uController, but says nothing about the driver.

I already know RS-232 lets you short circuit the TX & RX pins indefinitely (I've tested this and the current is limited to 30mA). Is there such protection for the SWDIO & SWCLK on the J-Link?

J-Link will not be damaged if this happens. If your target is damaged depends on the series resistors used to protect the target.

It seems the JLink is smart enough to disable the outputs when not in use, so then usually, there should be no short circuits. The LPC810M pins can source 45mA or sink 50mA, so I'm not worried, especially since they're only ~ $1.

Thank you Segger for designing it right and saving me from having to plug/unplug stuff for debugging

No, you didn't misunderstand. Although rare, it's quite possible I forget to reprogram the pin from GPIO to SWD in software before debugging, thus causing a short. But it's good to see 2 levels of protection.

I'm almost certain this has happened at least once, and my J-Link still works.