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OMAP-L138 External clock connection

David Meixner
Expert 4010 points
Other Parts Discussed in Thread: OMAP-L138

The OMAP-L138 datasheet (Figure 6-6) shows OSCVss connected to an external crystal through capacitors. However, the EVM schematic shows this pin connected directly to ground.

Additionally, the footnote in section 5.2 states

"When an external crystal is used oscillator (OSC_VSS, RTC_VSS) ground must be kept separate from other grounds and connected directly to the crystal load capacitor ground. These pins are shorted to VSS on the device itself and should not be connected to VSS on the circuit board. If a crystal is not used and the clock input is driven directly, then the oscillator VSS may be connected to board ground."

Can someone clarify how an external oscillator should be connected, including OSCVss? Is the EVM incorrect and still just happens to work?

Thanks,
David

  • 0
    TI__Guru 62975 points

    David,

    The datasheet recommendations should be followed.  The next major revision of the EVM is expected to change the oscillator circuit to match the datasheet.

    Using the OSCVSS pin is intended to reduce noise in the oscillator circuit.  Connecting the crystal to digital ground will not break the behavior outright, but using the OSCVSS pin is recommended.

    -Tommy

  • 0 in reply to tlee
    Prodigy 65 points

    We are planning to use a 4-pad crystal such as the 7M series of TXC (http://www.txccorp.com/download/products/c/7M-2008-P08.pdf). Should the two GND pads be connected to OSCVSS or VSS?

    Thanks,

    Hans