Hello,
I am new to Phy controllers and would have a couple of questions related to the LAN8710a on the ESP32 PoE Iso Schematic:
(https://i.imgur.com/ClJfdmF.png)
- In the iso version of the board, the Phy IC uses the ESP32 clock at CLKIN. For the non-Iso version, a quartz was used for clock generation. Why has that changed?
- Why are the bypass capacitors C12 and C17 different than the values recommended in the datasheet (47uF and 470pF respectively)?
- Why are 2.2k resistors used for the strapping pins instead of the 4.7k recommended by the LAN8710A eval board?
1. We always had ESP32-PoE and ESP32-PoE-ISO without external quartz. Since first releases. We used to have external quartz in ESP32-EVB, ESP32-GATEWAY to save pins. We still have external quartz in ESP32-EVB because we don't have any free pins there. However, we removed it in latest revisions of ESP32-GATEWAY. External quartz saves pins but adds other problems and compatibility issues with other designs.
2. and 3. optimizations of the hardware manufacturing that lead to lower number of different resistors and capacitors used; if there is just one 47uF resistor we have to use additional feeder in the pick and place machines; if you are a beginner in hardware design stick with the recommended values.
Just to remind you ESP32-PoE-ISO is CE certified board, with no noise problems:
https://olimex.wordpress.com/2019/07/08/esp32-poe-iso-passed-redemc-helth-and-safety-tests-necessary-for-issue-of-ce-marking/
Thank you for the calrification.
One more question: The fets FETs FET3 and T1 switch 3.3V to the Phy via GPIO12/PHY_PWR. Why is this necessary and why can't the 3.3V be directly applied to the phy (like on the ESP32-Gateway)? Is it because ESP32 MAC needs to initialize before the phy?
QuoteWhy is this necessary and why can't the 3.3V be directly applied to the phy (like on the ESP32-Gateway)? Is it because ESP32 MAC needs to initialize before the phy?
Yes, it is one of the reasons. Usually the ESP32 should start first then it can handle the Ethernet. Also the ESP32 should have control over the Ethernet. You might want to power off or reset the Ethernet portion of the board via software means. If you experience problems with the Ethernet you can toggle the GPIO.