5 years, 3 months ago.

MAC address on STM32F7 can't be changed?

Hello there,

I'm currently working on a project where we use a RMII/WiFi Bridge to connect a STM32F7 MCU to a wireless module for network connectivity. The exact MCU is a STM32F769NI Discovery Board.

The Situation

Since the WiFi modules we use come shipped with 4 individual MAC addresses for our usage (whereas Mbed generates them from the STMs UID, as can be seen in
mbed-os\features\netsocket\emac-drivers\TARGET_STM_EMAC\stm32xx_emac.cpp:mbed_mac_address())

we decided to implement the WEAK mbed_otp_mac_address() to overwrite this default MAC with one provided by the wireless module. Now I have found all of these various posts ( #1, #2, #3, #4 ) spead across the Internet, that's why we looked into mbed_otp_mac_address() in the first place.

Our solution

Here is our code:

#define UBLOX_SERIAL_TX PC_12
#define UBLOX_SERIAL_RX PD_2
#define UBLOX_BAUDRATE 115200

UARTSerial uart_ublox(UBLOX_SERIAL_TX, UBLOX_SERIAL_RX, UBLOX_BAUDRATE);
ATCmdParser* at_parser_ublox = new ATCmdParser(&uart_ublox, "\r", 256, 4000, false);

/**
 * @brief Return the MAC address in mbed.
 *
 * The address is read from the Flash register in the STM.
 *
 * @param mac  Char array to write the MAC address into
 * @return 1 if successful.
 */
uint8_t mbed_otp_mac_address(char *mac) {
    /* Reads the MAC address from an attached ublox NINA module. */
    /* This is the attached wireless module */
    at_parser_ublox->send("AT+UMLA=2");
    if (at_parser_ublox->recv("+UMLA:%2X%2X%2X%2X%2X%2X\r\nOK",
        &mac[0], &mac[1], &mac[2], &mac[3], &mac[4], &mac[5])) {
            printf("MAC set\n");

            return 1;
    } else {
    /* Return some fake MAC address.
     * The default for STM32F769NI is:  00:80:e1:1b:00:24
     */
        mac[0] = 0x00;
        mac[1] = 0x80;
        mac[2] = 0xe1;
        mac[3] = 0x1b;
        mac[4] = 0x00;
        mac[5] = 0x28;

        printf("Set default MAC\n");
        return 1;
    }
};

The Diagnosis

I have confirmed that the MAC address from the wireless module is read correctly and also returned properly.
I have also made sure that the function is called upon creating an Ethernet Interface.

Furthermore I have verified that the MAC address entered above is the exact same that EthernetInterface::get_mac_address() returns to me.
Additionally, as seen in the reference manual, Rev. 4, page 1849, the MAC address of the STM32F7 MCU is saved in a register (ETH_MACA0xx) and compared to the MAC address of incoming packets.

This bit of code shows me that the MAC address is indeed written into the register (After an Ethernet Interface has been initialized and EthernetInterface::connect() has been called!!):

void print_mac_reg() {
#if defined (TARGET_DISCO_F769NI)
        uint32_t ETH_MAC_ADDR_REG_OFFSET = 0x40UL;   /* RM0410, Rev.4, p. 1849 */
#else
#error "Cannot determine position of the MAC address register!"
#endif

    uint32_t *ETH_MAC_ADDR_REG_POS = (uint32_t *)(ETH_MAC_BASE + ETH_MAC_ADDR_REG_OFFSET);
    printf("[DEBUG ]: At 0x%08X : %08X\n", (ETH_MAC_ADDR_REG_POS + 0), *(ETH_MAC_ADDR_REG_POS + 0));
    printf("[DEBUG ]: At 0x%08X : %08X\n", (ETH_MAC_ADDR_REG_POS + 1), *(ETH_MAC_ADDR_REG_POS + 1));
}

All of this leads me to assume that changing the MAC address on the STM32F7 has worked.

The Problem

However, the STM32F7 now doesn't recognize packets adressed to it anymore (it seems to me).

If I leave the STM with it's default MAC address (00:80:e1:1b:00:24), it answers ICMP (ping) requests.
As soon as I change the MAC address (to 00:80:e1:1b:00:28 for example, or even something entirely different), it seemingly doesn't receive packets anymore. Calling ping now results in silence, the STM doesn't even answer ARP requests.

Calling nping (after changing the MAC address) with the --dest-mac option set to either the default value or the overridden value set by myself results in silence as well. If I make the STM send UDP packets to my host PC I can see in Wireshark that:

• The STM sends an ARP request
• My host PC answers this ARP request correctly
• No UDP packet is ever sent
• The STM sends more ARP packets indefinitely, which
• My host PC keeps answering, indefinitely...

Note, however, that Wireshark reports the correct Ethernet Frame address (the one returned to me by mbed, too).

This leads me to assume that the underlying hardware or low-level drivers somehow haven't been notified of the change in the MAC address.

Does anybody have an idea what might be wrong with this? Is somebody else having this problem or can someone else reproduce it? So far I could reproduce this issue across multiple F7-DISCO Boards.

More information

• IDE: VSCode, Version 1.37.1
• OS: Windows 10 Enterprise, Version 1803, OS-build: 17134.950
• Toolchain: GCC_ARM, Version 6, Release 2017 Q2
• Board (Target): STM32F769NI Discovery
• mbed-cli: Version 1.8.3
• mbed-os: checked out at Commit #1bf6b20df9d3cd5f29f001ffc6f0d0fcbbb96118

The full sample code

#include "mbed-os/mbed.h"
#include "Ethernet.h"
#include "EthernetInterface.h"


#define UBLOX_SERIAL_TX PC_12
#define UBLOX_SERIAL_RX PD_2
#define UBLOX_BAUDRATE 115200

UARTSerial uart_ublox(UBLOX_SERIAL_TX, UBLOX_SERIAL_RX, UBLOX_BAUDRATE);
ATCmdParser* at_parser_ublox = new ATCmdParser(&uart_ublox, "\r", 256, 4000, false);

/**
 * @brief Return the MAC address in mbed.
 *
 * The address is read from the Flash register in the STM.
 *
 * @param mac  Char array to write the MAC address into
 * @return 1 if successful.
 */
uint8_t mbed_otp_mac_address(char *mac) {
    /* Reads the MAC address from an attached NINA module. */
    at_parser_ublox->send("AT+UMLA=2");
    if (at_parser_ublox->recv("+UMLA:%2X%2X%2X%2X%2X%2X\r\nOK",
        &mac[0], &mac[1], &mac[2], &mac[3], &mac[4], &mac[5])) {
            printf("MAC set\n");

            return 1;
    } else {
    /* Return some fake MAC address.
     * The default for STM32F769NI is:  00:80:e1:1b:00:24
     */
        mac[0] = 0x00;
        mac[1] = 0x80;
        mac[2] = 0xe1;
        mac[3] = 0x1b;
        mac[4] = 0x00;
        mac[5] = 0x28;

        printf("Set default MAC\n");
        return 1;
    }
};

void print_mac_reg() {
#if defined (TARGET_DISCO_F769NI)
        uint32_t ETH_MAC_ADDR_REG_OFFSET = 0x40UL;   /* RM0410, Rev.4, p. 1849 */
#else
#error "Cannot determine position of the MAC address register!"
#endif

    uint32_t *ETH_MAC_ADDR_REG_POS = (uint32_t *)(ETH_MAC_BASE + ETH_MAC_ADDR_REG_OFFSET);
    printf("[DEBUG ]: At 0x%08X : %08X\n", (ETH_MAC_ADDR_REG_POS + 0), *(ETH_MAC_ADDR_REG_POS + 0));
    printf("[DEBUG ]: At 0x%08X : %08X\n", (ETH_MAC_ADDR_REG_POS + 1), *(ETH_MAC_ADDR_REG_POS + 1));
}



int main() {
    EthernetInterface eth;
    bool success = false;

    /* In my test scenario the host PC is the gateway in it's own private network
     * 192.168.0.0/24, whereas the STM is a client with the IP 192.168.0.11 
     */
    success = eth.set_network("192.168.0.11", "255.255.255.0", "192.168.0.1");
    if (success == NSAPI_ERROR_OK) {
        debug("Address set!\n");
    }

    success = eth.connect();
    if (success == NSAPI_ERROR_OK) {
        debug("Interface is connected!\n");
    }

    printf("Interface MAC address is %s\n", eth.get_mac_address());
    printf("The interface IP address is '%s'\n", eth.get_ip_address());

    /* Setup UDP so we can send out packets and check wireshark for the MAC */
    UDPSocket sock;
    UDPSocket *client_sock;
    SocketAddress remote("192.168.0.1", 10234);
    char *message = "Hello World!";

    sock.open(&eth);
    sock.connect(remote);
    sock.bind(10234);

    client_sock = &sock;

    while(1) {
        printf("[ INFO ]: Interface MAC address is %s\n", eth.get_mac_address());
        print_mac_reg();
        client_sock->send(message, strlen(message));
        wait(1);
    }
}