---
title: "W5500 stops to respond to ICMP after writing MSS register"
url: "https://maker.wiznet.io/forum/14136"
markdown_url: "https://maker.wiznet.io/forum/14136/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "kbch"
created: "2021-02-01T22:01:32+09:00"
last_activity: "2021-02-01T23:42:58+09:00"
language: "en"
views: 364
replies: 1
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# W5500 stops to respond to ICMP after writing MSS register

## Question

Asked by kbch on 2021-02-01 in Ethernet Chips.

Hi

I’m in the process of writing some code for the W5500. Currently, I’m facing the issue that as soon as I start to set the MSS the chip stops to respond to ICMP. Is this normal?

My code:

```
void w5500_read_data(uint16_t offset, uint8_t block, unsigned char *data_out, int length) {
    unsigned char *buffer = malloc(3 + length);
    
    buffer[0] = offset >> 8;
    buffer[1] = offset & 0xFF;
    buffer[2] = (block&0xF8) << 3;

    SpiOpenPort(0);
    SpiWriteAndRead(0, buffer, buffer, 3 + length, 0);
    SpiClosePort(0);

    memcpy(data_out, buffer + 3, length);
}

void w5500_write_data(uint16_t offset, uint8_t block, unsigned char *data, int length) {
    unsigned char *buffer = malloc(3 + length);

    buffer[0] = offset >> 8;
    buffer[1] = offset & 0xFF;
    buffer[2] = (block&0xF8) << 3;
    buffer[2] |= 0x04;

    memcpy(buffer + 3, data, length);

    SpiOpenPort(0);
    SpiWriteAndRead(0, buffer, buffer, 3 + length, 0);
    SpiClosePort(0);
}

void w5500_init(void) {
    unsigned char buffer0[1];

    // SW Reset
    buffer0[0] = 0x80;
    w5500_write_data(0x0000, 0x00, buffer0, 1);

    usleep(100 * 1000);
    
    w5500_read_data(0x0039, 0x00, buffer0, 1);

    if(buffer0[0] == 0x04) {
        printf("Detected Chip W5500! \n");
    } else {
        exit(1);
    }
}

void w5500_configure_ip(void) {
    unsigned char buffer0[6];

    // Write Gateway IP Address Register
    buffer0[0] = 172;
    buffer0[1] = 16;
    buffer0[2] = 90;
    buffer0[3] = 1;
    w5500_write_data(0x0001, 0x00, buffer0, 4);

    // Write Subnet Mask Register
    buffer0[0] = 255;
    buffer0[1] = 255;
    buffer0[2] = 255;
    buffer0[3] = 0;
    w5500_write_data(0x0005, 0x00, buffer0, 4);

    // Write Source Hardware Address Register
    buffer0[0] = 0x00;
    buffer0[1] = 0xAA;
    buffer0[2] = 0xBB;
    buffer0[3] = 0xCC;
    buffer0[4] = 0xDE;
    buffer0[5] = 0x02;
    w5500_write_data(0x0009, 0x00, buffer0, 6);

    // Write Source IP Address Register
    buffer0[0] = 172;
    buffer0[1] = 16;
    buffer0[2] = 90;
    buffer0[3] = 150;
    w5500_write_data(0x000f, 0x00, buffer0, 4);
}

void w5500_configure_socket(void) {
    unsigned char buffer0[2];

    // Write Socket n Maximum Segment Size Register - 1460
    buffer0[0] = 0x05;
    buffer0[1] = 0xB4;
    w5500_write_data(0x0012, 0x01, buffer0, 2);

    // Write Socket n Source Port - 5000
    buffer0[0] = 0x13;
    buffer0[1] = 0x88;
    w5500_write_data(0x0004, 0x01, buffer0, 2);
}

void w5500_connect_socket(void) {

}

int main(void) {
    w5500_init();
    w5500_configure_ip();

    w5500_configure_socket();
    //w5500_connect_socket();
}
```

## Replies

### Reply 1 by kbch, 2021-02-01

Ok… Layer8 issue… `buffer[2] = (block&0xF8) << 3;` should be `buffer[2] = (block&0x1F) << 3;`

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Source: https://maker.wiznet.io/forum/14136