#include "USBHostMSD.h"
 
DigitalOut led1(LED_GREEN);
 
int main() {
    USBHostMSD msd("usb");
    if (!msd.connect()) {
        error("USB Flash drive not found.\n");
    }    
    FILE* fp = fopen("/usb/test1.txt", "a");
    if (fp) {
        fprintf(fp, "Hello from mbed.\n");
        for(int i = 0; i < 21; i++) {
            fprintf(fp, " %d", i);
            led1 = !led1;
        }
        fprintf(fp, "\n");
        fclose(fp);
    }
    fp = fopen("/usb/test1.txt", "r");
    if (fp) {
        int n = 0;
        while(1) {
            int c = fgetc(fp);
            if (c == EOF) {
                break;
            }
            printf("%c", c);
            n++;
            led1 = !led1;
        }
        fclose(fp);
        printf("%d bytes\n", n);
    }
 
    while(1) {
        led1 = !led1;
        wait_ms(200);
    }
}

I used the USB host library from Norimasa Okamoto ( https://developer.mbed.org/users/va009039/code/F401RE-USBHost/) for my FRDM K64 board and then a simple app to test the MSD. When I insert the USB stick into the OTG port, I do not get any interrupts and no interrupt function is called. I tried with USB keyboard,but the same result. I keep getting "Please attach USB device". Never the interrupt function got called USBHALHost::UsbIrqhandler(); used the same code as in the example(main.cpp of F401RE-USBHostMSD_HelloWorld) as below <<code title=main.cpp snippet>> #include "USBHostMSD.h" DigitalOut led1(LED_GREEN); int main() { USBHostMSD msd("usb"); if (!msd.connect()) { error("USB Flash drive not found.\n"); } FILE* fp = fopen("/usb/test1.txt", "a"); if (fp) { fprintf(fp, "Hello from mbed.\n"); for(int i = 0; i < 21; i++) { fprintf(fp, " %d", i); led1 = !led1; } fprintf(fp, "\n"); fclose(fp); } fp = fopen("/usb/test1.txt", "r"); if (fp) { int n = 0; while(1) { int c = fgetc(fp); if (c == EOF) { break; } printf("%c", c); n++; led1 = !led1; } fclose(fp); printf("%d bytes\n", n); } while(1) { led1 = !led1; wait_ms(200); } } <</code>> Also for FRDM K64 the initialisation code also contains MPU->CESR=0; Why did not this work. Any clues or suggestions? -yk