eliminate dedicate TEST_MODE pin？

[david-edaboard]

how to eliminate the dedicate TEST_MODE pin of the chip, and share it with a normal function pin? for test mode I can pull it to certain value to enter test mode.
Do I need to use a latch to do this?
thanks!

 

[dick_freebird]

You can make custom I/Os that have more than two valid input states. Or find a combination of other control, static inputs that ought to never coincide, and decode. Or hijack an unused register bit that can be set by a "test preamble" via an exposed data link.

A special-case below-vss or above-vdd sniffer can be superimposed on a CMOS-logic input with little penalty but these can be noise sensitive, might need some filtering / latching / hysteresis.