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PicoZed Hardware Design eMMC breakout traces
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eMMC breakout traces

Former Member
Former Member over 8 years ago

We are designing a picoZed carrier board with an eMMC chip and are using the same eMMC as the picoZed board.


The eMMC on the picoZed has three signals that are shown on the schematic as 'break out' signals that connect from eMMC NC (No Connect) pins to eMMC functional pins. These eMMC NC pins and corresponding connection eMMC functional pins  are shown below, where the dashed line represents the breakout connection.


NC (pin Y1) ---- GND (Y2)
NC (pin K1) ---- VDDIM (K2)
NC (pin J1) ---- DAT3 (J2), net EMMC_IO3


The following note is in the schematic.


Layout Note:
These unused pins shorted intentionally to break out of BGA.


Is the intent when routing the PCB to have an outer layer trace connecting each of the above pin pairs?


What is the purpose of these connections? Probing of these signals, I suspect, possibly for characterization?


As an aside, the eMMC part number listed on the picoZed schematic and BOM is a Micron MTFC4GMDEA-4MIT in a 153 pin BGA package. However, the pin numbers on the schematic and actual PCB are for a 169 pin BGA package. I believe this is done to allow a higher capacity eMMC chip - which is only available in a 169 pin package - to be used on the SOM. The 153 pin package is a subset of the 169 pin, and the standard build of the PicoZed has the 153 pin package installed. I couldn't find this information on the schematic or user guide. If this is documented would you please point me to it?

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  • jafoste4
    0 jafoste4 over 8 years ago

    Hello Urbite,

    1)     Is the intent when routing the PCB to have an outer layer trace connecting each of the above pin pairs?

    We utilize the NC pins on the eMMC packages to facilitate layout of the interface with common manufacturing processes. We utilize the NC pins to give us a route out of the BGA pattern. If we didn’t do this, we would have to use more advanced techniques which equates to a more expensive PCB that is harder to fabricate. It’s common for the 0.5mm pitch BGAs.

    2)     As an aside, the eMMC part number listed on the picoZed schematic and BOM is a Micron MTFC4GMDEA-4MIT in a 153 pin BGA package. However, the pin numbers on the schematic and actual PCB are for a 169 pin BGA package. I believe this is done to allow a higher capacity eMMC chip - which is only available in a 169 pin package - to be used on the SOM. The 153 pin package is a subset of the 169 pin, and the standard build of the PicoZed has the 153 pin package installed. I couldn't find this information on the schematic or user guide. If this is documented would you please point me to it?

     The eMMC footprints on PicoZed (both PCB flavors) support higher density eMMC through a shared footprint. If you look at PicoZed PCBs you will see an outline that represents the larger package associated with the higher density eMMC devices. We have not populated the larger device package as this was meant as a customization feature and I don’t believe anyone has asked for it yet so we haven’t proactively validated it.

    --Josh

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  • Former Member
    0 Former Member over 8 years ago in reply to jafoste4
    We utilize the NC pins on the eMMC packages to facilitate layout of the interface with common manufacturing processes. We utilize the NC pins to give us a route out of the BGA pattern. If we didn’t do this, we would have to use more advanced techniques which equates to a more expensive PCB that is harder to fabricate. It’s common for the 0.5mm pitch BGAs.

    Ah, makes sense now. The signals of interest are on the second column of balls and the leftmost column 1 is NC for those balls. So you route from signal pin n2 to NC pin n1 and the signal is outside of the BGA. Very clever!

    The eMMC footprints on PicoZed (both PCB flavors) support higher density eMMC through a shared footprint. If you look at PicoZed PCBs you will see an outline that represents the larger package associated with the higher density eMMC devices. We have not populated the larger device package as this was meant as a customization feature and I don’t believe anyone has asked for it yet so we haven’t proactively validated it.

    Yes, I saw on the SOM that the 4GB is actually installed but the silkscreen is for the larger footprint. It was a bit puzzling that the schematic shows the eMMC to be a specific part number, but then the schematic pin numbers don't match the given part number. If you rev the schematic it would be nice to have a note on the schematic explaining this.

     

    Thanks for feedback and explanation, Josh

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