I am unable to initialize a W5100 module using an Arduino Pro Mini. I try setting the MAC and IP addresses but read invalid values thereafter.

Here is the Arduino program along with the output:

void setup() {
    uint8_t mac[] = {0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED};
    uint8_t local_ip[] = {192, 168, 1, 100};
    Ethernet.begin(mac, local_ip);
    printMacAddress(); // OUTPUT: FF:FF:FF:FF:FF:FF
    printIpAddress(); // OUTPUT:

void printMacAddress() {
    uint8_t mac[6];
    Serial.print(mac[0], HEX); Serial.print(":"); Serial.print(mac[1], HEX); Serial.print(":");
    Serial.print(mac[2], HEX); Serial.print(":"); Serial.print(mac[3], HEX); Serial.print(":");
    Serial.print(mac[4], HEX); Serial.print(":"); Serial.print(mac[5], HEX); Serial.println();

void printIpAddress() {
    IPAddress ip = Ethernet.localIP();

I read in some blog posts that the Arduino may initialize before the W5100 initialization is completed so others recommend manually resetting the W5100 after Arduino initialization. So I connected the W5100 reset pin to the Arduino digital output 4 and reset the W5100 during setup using the following code.

const int W5100_RESET_PIN = 4;
pinMode(W5100_RESET_PIN, OUTPUT);
digitalWrite(W5100_RESET_PIN, LOW); // Activate reset pin by asserting low
delay(5); // Wait for 5 msec
digitalWrite(W5100_RESET_PIN, HIGH); // Deactivate reset pin by asserting high
delay(300); // Wait for W5100 to initialize[/code]

I observed the W5100 resetting because the module LEDs flashed off and back on each time I manually reset the Arduino. However this does not fix the W5100 mac/IP initialization issue.

How should I troubleshoot this issue?

Here are pictures of the assembly. https://i.sstatic.net/cjnHE.jpg

Here is an overview of the W5100 module. http://arduino-info.wikispaces.com/Ethernet

1 Answer 1


One of the Ethernet examples shows you should test the result of the begin function call, like this:

// start the Ethernet connection:
  if (Ethernet.begin(mac) == 0) {
    Serial.println("Failed to configure Ethernet using DHCP");
    // try to congifure using IP address instead of DHCP:
    Ethernet.begin(mac, ip);

Try displaying the return value from begin to see if that initialization succeeded.

  • Thanks for your comment Nick. Calling Ethernet.begin(mac) returns non-zero and it fails to configure DHCP. I also tried configuring IP manually and this fails as well.
    – user16534
    Commented Jan 20, 2016 at 22:07

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