I am designing a PCB which uses LAN7850 chip

This is the first time I'm using such a chip and I'm very newbie to PCB designing.

Please advise if this Chip requires a GPIO pin from the microcontroller to control the RESET_N as I have done in this image.


Design Rule Checks:

enter image description here

  • \$\begingroup\$ what's that mosfet doing in the reset signal line? why? \$\endgroup\$ – Marcus Müller Apr 24 at 21:07
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    \$\begingroup\$ It's a good idea to have the reset line tied with a "board level" reset otherwise you won't be able to reset the chip without a board power cycle. Also because of timing requirements, you may have to reset this chip after the MCU/MPU starts up, or hold it in reset until a power rail comes up. Are you using this chip in USB mode, or HSIC? \$\endgroup\$ – Ron Beyer Apr 24 at 21:33
  • \$\begingroup\$ Mosfet simulates a button press so I'm using it to remove the issue of interference and the influence of levels. Ron - I am using it in USB mode. So it is always the best option to connect a GPIO? \$\endgroup\$ – Arahasya Apr 24 at 21:50
  • \$\begingroup\$ Also, it is recommended to use 4 layers for ethernet but I am able to fit my components in 2 layers with all the rule checks. Do I proceed with this 2 layers? \$\endgroup\$ – Arahasya Apr 24 at 21:52
  • \$\begingroup\$ Yes, I would have it. You should also have a 4 layer board, with the two inner layers being ground and power. This is important. You also need to pay attention to USB and RX/TX pair differentials/matched lengths and they really need to be routed above a reference plane. \$\endgroup\$ – Ron Beyer Apr 24 at 21:53

It isn't necessary as USB has a provision built into the protocol for the host to initiate a reset. This is how it would be used if, for example, the device were a USB-to-LAN dongle.

However, you should have a reset to the IC that is held down until after the LAN7850 power becomes stable.

As a nice-to-have, OR-ing the power-on reset with a GPIO reset might be useful for testing or for power management.

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  • \$\begingroup\$ thanks for your inputs \$\endgroup\$ – Arahasya Apr 24 at 22:08

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