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DP83848M: 25Mhz (Pin 21) Clock Out

Part Number: DP83848M
Other Parts Discussed in Thread: DP83848H

Hi community,

Our team is having some trouble correctly understanding the function of pin 21 on the DP83848M ethernet phy transceiver. We are confused whether there is a on-die oscillator, and pin 21 can be used as clock, or whether an external clock source should be connected to the clock input pin (28) and the 25 Mhz clockout pin ist just the output of the buffered clock input and could be used as clock input to a second PHY.
We have probed the pin, and for sure, there is no clock. However, we would like to make sure that our understanding is correct.

Thank you in advance,


  • Hello Liliane,

    Pins 28 and 27 is where an external crystal or oscillator should be connected. A crystal would use both pins whereas an oscillator should be connected to 28 only (27 left floating).

    Pin 21 is a CLK_OUT pin ony on the DP83848H/M/T variants and not the J/K. Please double check that you have the right part. This CLK_OUT can be used as an input clock for another device or the MAC.