PDM II won't boot without microSD card

PDM II won't boot without microSD card

The PDM II will not boot without a microSD card. The mainboard LEDs will light, but nothing else happens. The user has likely had the cover open and used the microSD card to initialize the unit. The unit will still boot with a microSD card, but not without one.

Overview

The card detect switch on the microSD card carrier is bent and stays closed even when there is no microSD card. The mainboard thus always tries to boot from the microSD card, but if one isn't present it fails. The unit can be used if a microSD card is left in the carrier, but in that case it will reset to factory defaults every time it is power-cycled. The switch must be kept open before the unit can boot without a card.

Details

The mainboard has a microsSD card carrier, used to initialize the unit when it can't be upgraded via the web UI. There is a pair of contacts on one side that detect the presence of a card, closing when the card is present. This tells the mainboard to boot from the card, rather than the built-in flash memory.

If these contacts become bent in the process of inserting or removing a microSD card, they can stay closed even without a card being present. Then, when the unit is booted, the CPU tries to read from a microSD card when there's none present, and the boot fails.

Solution

The long-term solution is to repair or replace the mainboard. However, there's an easy short-term solution that will allow the unit to reliably start up and run.

Take a sheet of printer paper, and cut a 1/8" x 1/4" (3mm x 8mm) strip. Holding the strip with tweezers, slide it between the contacts to prop them open.

This will tell the mainboard to boot from the built-in flash, which should have been initialized the last time the unit successfully booted from a microSD card.

Confirm that the unit boots properly. Then, carefully close the card carrier against the paper, and test it again. Finally, replace and fasten the unit's cover. The paper is so light that it is unlikely to be dislodged, so this should be a long-term solution.