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I have a board designed for a dual-die Micron MT25QL512 QSPI NOR flash. These parts are in short supply, but the board fabricator had some single-die MT25QL128, and used them instead. The single-die chip has the same footprint as the dual-die chip, but the pins for the second die are marked 'RFU' in the datasheet, with the note:

"Reserved by Micron for future device functionality and enhancement. Recommend that these be left floating. May be connected internally, but external connections will not affect operation."

Is there any risk in just leaving the weak pull-ups on the unused pins? (On the MCU I'll mux the pins to GPIO input).

UPDATE: It seems to be okay just to leave the pull-ups on the unused pins. I've been running for 3 weeks like that, without noticing anything strange.

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  • \$\begingroup\$ This is so not off topic \$\endgroup\$
    – DKNguyen
    Dec 12, 2019 at 2:28

1 Answer 1

Reset to default
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These pins can have any number of uses. Sometimes they enable internal test modes or other funky connections and can cause malfunctioning of the device

In one possible case given the information provided, they may be binning dual-die devices to single die when there is a failure or defect in one die instead of scrapping all together. This may leave some internal structure/electrical connections in tact, but this may not be consistent if they still run dedicated single-die production, the pins are reserved to indicate the potential for oddness.

The key is that

Reserved by Micron for future device functionality and enhancement. Recommend that these be left floating. May be connected internally, but external connections will not affect operation.

It means they are giving you the go-ahead that you will not break the IC with external connections as long as they respect the absolute maximum ratings which apply to all the pins, but that doesn't mean that external connections may not cause electrical conflicts/shorts/additional current draw in certain drop-in applications.

Likely leaving a weak pull-up will have no long term effect as you have verified/tested.

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