I have a small simple board with 4 push buttons. The 4 lines are pulled hi in the micro and pushing the PB should pull each low independently. However, On 5 out of 16 new production boards when I push the MENU button the unit acts like I also pressed the ENTER button. I can see the ENTER line drop from 3.3V to 1.4V like there is a short between J4-1 and J4-2 but I don't measure a short. I also see the PLAY line get pulled down to ~3.1V when MENU is pushed. On the working units these lines are steady 3.3V. If I measure from the PB1-1 to J4-1 I see 33 ohms and same for PB2-1 to J4-2 so the resistor network IC seems fine. The good board measures Mohms from J4-1 to J4-2 and the bad boards are measuring about 200k!

The scm section in the dotted box is the push button board. The mating connector and ESD protection is shown for completeness.

push button board scmpcb

I am suspecting board contamination may be the cause but didn't think board flux/residue would so drastically affect such a non-sensitive circuit.

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  • \$\begingroup\$ How weak are the pullups in the micro? \$\endgroup\$ – Spehro Pefhany Sep 25 '15 at 13:47
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    \$\begingroup\$ Wash a bad board in flux remover. \$\endgroup\$ – Brian Drummond Sep 25 '15 at 14:24

In a clean design even a 200k switch open resistance should not cause malfunction. Do you use external or uC internal pull-ups? In case of internal pull-ups, what voltage will you get if you calculate with 200k switch open resistance and lowest supply voltage? Is this voltage still be in logic-high range?

There is no such thing as a perfect switch/button. I usually calculate with 100 Ohm closed and 100k open.

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