How to strap the LAN8720A pins?

In most applications, you want to set MODE[2:0] to 111 in order to enable the all-capabilities auto-negotation mode. The other modes are rather special and rarely used (such as repeated mode). In effect, this means:

• Strap RXD0/MODE0 high – it has an internal pull-up so you don’t need an explicit strap
• Strap RXD1/MODE1 high – it has an internal pull-up so you don’t need an explicit strap
• Strap CRS_DV/MODE2 high – it has an internal pull-up so you don’t need an explicit strap

In applications where you don’t have an external 1.2V regulator, you need to strap REGOFF=0, i.e.:

• If you don’t use an LED, strap REGOFF/LED1 low – it has an internal pull-down, so you don’t need an explicit strap.
• If you use an LED on LED1, you want the anode of the LED to be connected to the LAN8720A (with the cathode of the LED being connected to GND using a current-limiting resistor)

The last strap you need to think about is nINTSEL/REFCLKO. You need to evaluate whether you need the REFCLKO pin to output a 50MHz reference signal in your application. This is only the case if you feed the REFCLKO output to a clock input of another IC.

• In case you need nINT/REFCLKO to output the 50 MHz reference clock, you need to strap LED2 low. You need an explicit strap to do so, or in case you are using an LED on LED2, you want the anode of the LED to be connected to the LAN8720A (with the cathode of the LED being connected to GND using a current-limiting resistor), and a 10K resistor in parallel to the LED.
• In case you need nINT/REFCLKO to act as an interrupt pin, (or if you don’t care), you need to strap LED2 high.  Since LED2 has an internal pull-up, you don’t need an explicit pull-up. or in case you are using an LED on LED2, you want the cathode of the LED to be connected to the LAN8720A (with the anode of the LED being connected to 3.3V using a current-limiting resistor).

Summary

For the LAN8720A, you often don’t need explicit straps – the only pin where you have to seriously think for 95% of all PCB designs is whether you need the REFCLKO feature.

For further reference, see the LAN8720A datasheet.