Heisen Bug

#include "mbed.h"

#include "EthernetInterface.h"
// #include <UDPSocket.h>

#define IP "10.200.1.123"
#define MASK "255.255.255.0"
#define GATEWAY "10.200.1.1"
// #define LED_FREQ 400000 // spi

#define OPC_PORT 7890

volatile int count = 1;

uint8_t firmware_version = 42;
//uint8_t mac_addr_address = 0xFA;
typedef unsigned char _enet_address[6];
_enet_address mac_addr;

SPI spi(SPI_MOSI, SPI_MISO, SPI_SCK);
typedef struct
{
    const unsigned int start = 0;
} start_frame_t;

// a single LED frame
typedef struct
{
    unsigned char brightness;
    unsigned char blue;
    unsigned char green;
    unsigned char red;
} APARGB_t;

typedef struct
{
    const unsigned int end = 0xffffffff;
} end_frame_t;

typedef struct
{
    start_frame_t start_frame;
    APARGB_t LED_frames[LED_NUM];
    end_frame_t end_frame;
} APA_buffer_t;

APA_buffer_t sum_buf;

// @ NOTE 1: This block is mutually exclusive with runtime function of EthernetInterface
void transfer()
{
    for (int i = 0; i < LED_NUM; i++)
    {
        sum_buf.LED_frames[i].brightness = ((0x7 << 5) | LED_GLOBAL);
        sum_buf.LED_frames[i].red = (led_buf[i] & 0xff);
        sum_buf.LED_frames[i].green = ((led_buf[i] >> 8) & 0xff); // G
        sum_buf.LED_frames[i].blue = ((led_buf[i] >> 16) & 0xff); // B
    }
    spi.transfer((const void *)&sum_buf, sizeof(sum_buf), (void *)NULL, 0, bit_width, NULL, 0);
    // the following call also breaks
    //   spi.write((const char*)&sum_buf, sizeof(sum_buf), (char*)NULL, 0);

    // @ NOTE 2: I can use the following block no matter what but it is too slow for my application
    //   int i;

    //   // start frame
    //   for (i = 0; i < 4; i ++) {
    //     spi.write(0);
    //   }
    //   // led frame
    //   for (i = 0; i < LED_NUM; i ++) {
    //     spi.write((0x7 << 5) | LED_GLOBAL);
    //     spi.write(led_buf[i] & 0xff); // R
    //     // spi.write(packet.values[i].blue);
    //     spi.write((led_buf[i] >> 8) & 0xff); // G
    //     // spi.write(packet.values[i].green);
    //     spi.write((led_buf[i] >> 16) & 0xff); // B
    //     // spi.write(packet.values[i].red); // R
    //   }
    //   // end frame
    //   for (i = 0; i < 4; i ++) {
    //     spi.write(1);
    //   }
}

int main()
{
    int tswitch = 0;
    APA102_strip strip;
    strip.testStrip();

    EthernetInterface net;
    int network_status = net.set_network(IP, MASK, GATEWAY);
    if (network_status != 0)
    {
        printf("ERROR: Network Status: %d\n", network_status);
    }
    net.connect();
    const char *ip = net.get_ip_address();
    printf("IP address is: %s\n", ip ? ip : "No IP");

    while (tswitch < 7)
    {
        wait(0.05);
        if ((tswitch++) & 1)
        {
            strip.allRed();
            strip.transferStrip();
        }
        else
        {
            strip.allGreen();
            strip.transferStrip();
        }
    }

    // Open a socket on the network interface, and create a TCP connection to mbed.org
    UDPSocket socket;
    socket.open(&net);
    socket.bind(OPC_PORT); //port to use

    SocketAddress address(&net, NULL, NULL);

    // // OPC Message object
    OPC_message message;

    // @ NOTE 3: commenting or uncommenting the following line will determine whether or not SPI::transfer works at runtime
    socket.recvfrom(&address, &message.packet, sizeof message.packet);
};