---
title: "Debugging issues with W5500/LAN8720 communicating with STM32 and gigabit switch chips"
url: "https://maker.wiznet.io/Aimee0/projects/debugging-issues-with-w5500-lan8720-communicating-with-stm32-and-gigabit-switch-chips/"
markdown_url: "https://maker.wiznet.io/Aimee0/projects/debugging-issues-with-w5500-lan8720-communicating-with-stm32-and-gigabit-switch-chips/md"
type: "UCC: User Created Content"
author: "Aimee"
author_url: "https://maker.wiznet.io/Aimee0/"
original_author: "aimee"
original_url: "https://blog.csdn.net/tjcwt2011/article/details/143864610"
published: "2025-05-28"
language: "en"
likes: 1
views: 2353
comments: 0
source: "WIZnet Makers (https://maker.wiznet.io/)"
---

# Debugging issues with W5500/LAN8720 communicating with STM32 and gigabit switch chips

> Debugging issues

Original author: aimee (source: https://blog.csdn.net/tjcwt2011/article/details/143864610)

## Article

---

## **Troubleshooting Guide**

### **Subject**: W5500 / LAN8720 + STM32 + Gigabit Switch Communication Troubleshooting

---

## **1. W5500 Not Working: Check the Crystal OscillatorSymptoms**: W5500 does not operate
**Possible Cause**: External crystal not oscillating properly
**Checklist and Actions**:

- Use an oscilloscope to check the XTAL1/XTAL2 pins for a 25 MHz signal

- Valid signal characteristics:

- Around 1.5 V DC offset

- Approx. 0.5 V peak-to-peak AC amplitude

- Replace the crystal component if signal is not observed or soldering is poor

---

## **2. SPI Communication Issue Between STM32 and W5500Symptoms**: SPI communication unstable or completely failed
**Possible Cause**: Incorrect SPI pin setup or excessive SPI speed
**Checklist and Actions**:

- Set SPI mode to **AF_PP** (Alternate Function Push-Pull)

- Set GPIO output speed to **High (50 MHz)**

- If line length is long, test with a lower SPI clock (1–4 MHz recommended)

---

## **3. W5500 and Gigabit Switch Connection ProblemSymptoms**: No link established between W5500 and the switch
**Possible Cause**: Electrical level mismatch or lack of Auto-MDIX support
**Checklist and Actions**:

- Confirm the switch chip supports **Auto-MDIX**

- Ensure correct signal routing:

- W5500 TXP/TXN ↔ Switch RX

- W5500 RXP/RXN ↔ Switch TX

- Use a network transformer to match signal characteristics

---

## **4. UDP Reception Fails on PCSymptoms**: MCU sends UDP packets, but PC does not receive them
**Possible Cause**: Windows firewall or antivirus software blocking packets
**Checklist and Actions**:

- Temporarily disable firewall to test

- Alternatively, add a firewall exception for the specific UDP port

- If network is classified as Public, switch it to Private for local access

---

## **5. Gigabit Switch Does Not WorkSymptoms**: No activity, no link LED, no heat on the chip
**Possible Cause**: Component misplacement, especially pull-up/down resistors
**Checklist and Actions**:

- Double-check circuit diagram against the PCB

- Specifically verify resistors on the **MDIREF** pin (e.g., 2.49kΩ)

- Ensure proper voltage levels at the switch’s power and signal lines

---

## **6. LWIP UDP Socket Binding IssueSymptoms**: Only one remote IP/port can communicate via UDP
**Possible Cause**: UDP socket bound to a fixed remote address
**Checklist and Actions**:

- Use `IP_ADDR_ANY` when calling `udp_bind()` to allow multiple sources

- Avoid binding to a single remote IP/port unless required

---

## **7. Quick Summary of Checklist**

| No. | Issue | Action Summary |
| --- | --- | --- |
| 1 | W5500 crystal not oscillating | Check 25 MHz signal using oscilloscope |
| 2 | STM32–W5500 SPI unstable | Adjust pin mode, lower SPI clock |
| 3 | No link with gigabit switch | Ensure Auto-MDIX support, use transformer |
| 4 | UDP data not received on PC | Configure firewall or disable temporarily |
| 5 | Switch not powering on | Check component values and MDIREF resistor |
| 6 | UDP reception limited to one peer | Bind UDP socket to `IP_ADDR_ANY` |

---

## **8. Additional Best PracticesPower Stability**: Ensure proper and stable power to all devices.

**Firmware Updates**: Make sure STM32, W5500, and LAN8720 firmware are up to date.

**Visual Indicators**: Use LEDs to monitor link and activity states.

**Packet Monitoring**: Use tools like **Wireshark** to analyze actual UDP traffic and packet behavior.

---

Source: https://maker.wiznet.io/Aimee0/projects/debugging-issues-with-w5500-lan8720-communicating-with-stm32-and-gigabit-switch-chips/