Most of the Ethernet PHY chips (e.g. LAN8720) provide strapping option to control operation.

I am short of IO pins in my uC, and I do not need (AFAIK) to change anything more in the PHY operation.

(1) Can I just not use the MDIO interface (Just PU MDC/MDIO pins)?

(2) If not, What are the "required" operations I need to do with PHY?


  • \$\begingroup\$ 1. Look at the datasheet for the PHY. 2. Look at the things you can do with MDIO. 3. Do you need to do any of those things? \$\endgroup\$
    – user253751
    Mar 12, 2018 at 3:02
  • \$\begingroup\$ Problems may arise if you need to support both 10Mbps and 100Mbps auto-negotiation, as the link speed may not be properly signalled to the MAC. \$\endgroup\$
    – Grabul
    Mar 12, 2018 at 3:21
  • \$\begingroup\$ Good point. uC is slow anyway and will accept any speed. The PHY will autonegotiate with the other side. \$\endgroup\$
    – alsaleem
    Mar 12, 2018 at 3:37

1 Answer 1


By definition, "strapping option" is a configuration that is set in hardware by pulling up, down, or no pull on certain pins, which usually takes an effect only at release of hardware reset. This is done to save the number of package pins, which saves on costs. Therefore, there is usually no need to waste GPIO pins on that, unless you plan to do something totally crazy with communication channel.

The operations that are "required" requires you to select certain mode of operation (see Table 3-4 of datasheet), hardwire the straps accordingly, and stick to it. If you want some super-flexibility, you will need to control all straps and hardware reset pin to the PHY.

  • \$\begingroup\$ I see that most control operations (chapter 4) are not to important to my project needs. I was asking so someone confirms that these are not commonly used. Thanks. \$\endgroup\$
    – alsaleem
    Mar 12, 2018 at 3:31
  • \$\begingroup\$ After strapping, I do not need any further control. However, the only point that brings to my attention is software reset. Does PHY hang somehow? \$\endgroup\$
    – alsaleem
    Mar 12, 2018 at 3:40

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