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I am working on 5-port gigabit Ethernet switch based on Microchip KSZ9897. As the KSZ9897 is 7-port gigabit Ethernet switch (5 x 1000BASE-T/100BASE-TX/10BASE-Te IEEE 802.3 + 2 x RGMII/MII/RMI) and I am using only the 5 ports, I just wonder what to do with the remaining RGMII/MII/RMII unused interfaces (pins). Shall I put it unconnected? I didn't find anything from the documentation about the particular situation. I suppose the question does not depend much on chip manufacturer, but is related to unused RGMII/MII/RMII interface in general.

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  • \$\begingroup\$ somewhere in the datasheet it will say what to do ith unused IO pins. \$\endgroup\$ – Jasen Feb 10 '18 at 22:12
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If it can help to someone. After a while I got the answer from the Microchip technical support. It was unclear for me at the first as I didn't get concrete answer, but after a few iterations I came to the answer that it can be left unconnected if the EMI is not critical. After asking "What to do with unconnected pins if I want to be EMI aware", I got the suggestion to read the following links that can be helpful: Link1, Link2. I hope this may help someone withe the similar problem.

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If it doesn't specifically say what to do in the datasheet, I would contact Microchip (as you did) and see what they said...

Personally, if I could afford the board real-estate and BOM cost, I would place 0R resistor options from the unused inputs to GND. It's possible the chip designers placed internal pull-downs in the chip, but that is unlikely on high-speed lines such as a clock. Strapping an unused clock to GND would be a very safe option. Using a 10K or 100K resistor would cover you in the case of some kind of short-circuit path as it would limit current to milli or microamps.

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