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DP83822I: DP83822I use with PoE is NOT connected to Ethernet

Cuong Nguyen Viet
Prodigy 130 points
Part Number: DP83822I
Other Parts Discussed in Thread: TPS23731

Hi Team,

I have an Ethernet and PoE design using DP83822I and TPS23731, PoE can work with TP-Link Switch(TL-SG1005P), Ethernet cable uses 2 pairs of 1236 wire but the PHY IC seems not to work completely, not connected to Ethernet signal.
Currently the circuit is basically configured as follows (via Strap pins):
- Operation Mode:10BASE-Te, Half/Full-Duplex / 100BASE-TX, Half/Full-Duplex
- MAC Interface Configuration: RGMII, 25-MHz Reference Clock
- PHY Address: PHY_AD[4:0] = 00001
RGMII interface is connected to iMX8MP, logic level 1V8(VDDIO_ETH=1V8), after connecting to PoE Switch, LED green is ON/Blink, LED blue is ON.
I have reworked Termination at PoE transformer to look like the DP83822 EVM (removed R103, R104, C120).

I have a few questions as follows:
1. Does the transformer direction I use according to PoE EVM (CMC is not towards cable side) when using PoE affect the 3V3 source connected to the midpoint (CT inputs) of the transformer and the Ethernet signal?
2. In DS and EVM it is mentioned to use resistor values at MDIO(PU 2.2k) and RBIAS(4.87k), in my design use resistors at MDIO(PU 1.5k) and RBIAS(4.7k ), is there any formula to calculate these values? or how can I use other values within the bias range? Currently I can rework the 2.2k MDIO PU but I don't have the 4.87k resistor.
3. I use external crystal 25MHz at XI, XO, do I need to configure registers once I have configured the strap pin?

Please refer to the SCH attached.
Kindly review and let me know your valuable comments. Thanks a lot.



Viet Cuong
Hardware Engineer

  • 0
    TI__Genius 12616 points

    Hi Viet Cuong,

    Please find me responses below.

    Cuong Nguyen Viet said:
    1. Does the transformer direction I use according to PoE EVM (CMC is not towards cable side) when using PoE affect the 3V3 source connected to the midpoint (CT inputs) of the transformer and the Ethernet signal?

    No, it should not have an impact on the signal.

    Cuong Nguyen Viet said:
    2. In DS and EVM it is mentioned to use resistor values at MDIO(PU 2.2k) and RBIAS(4.87k), in my design use resistors at MDIO(PU 1.5k) and RBIAS(4.7k ), is there any formula to calculate these values? or how can I use other values within the bias range? Currently I can rework the 2.2k MDIO PU but I don't have the 4.87k resistor.

    We recommend 2.2 kΩ but 1.5 kΩ is okay, if you are able to read/write registers then you can keep the 1.5 kΩ. The RBIAS value of 4.87 kΩ ± 1% is very important. The 4.7 kΩ resistor is already outside the 1% range.

    Cuong Nguyen Viet said:
    3. I use external crystal 25MHz at XI, XO, do I need to configure registers once I have configured the strap pin?

     No, whether you're using a crystal or oscillator, no register writes are required. 

    Regards,

    Alvaro