Question

can anyone please explain this code?

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/signal.h>

char n = 0;
char FLAG =0x00;
char FLAG2 =0x00;

char RST=0x00;
unsigned char minutes_save [20];
unsigned char seconds_save [20];
int seconds, minutes, shift, count;




void init(void)
{

    DDRB = 0xff;
    DDRA  =0xff;
    MCUCR = 0x0F;
    GICR = 0xC0;
    TCCR2 = 0x05;
    ASSR = 0x08;
    TCNT2 = 0x00;
    sei();
}


SIGNAL(SIG_INTERRUPT0)

{   

    if (FLAG == 0x00)
        TIMSK = 0x40;

    if (FLAG == 0x01)
        TIMSK = 0x00;

    FLAG = FLAG ^ 1;

}

Answer 1

Whenever the program receives an interrupt signal, it modifies the value of TIMSK to be 0x40 (64 in decimal) or 0x00 (0 in decimal) depending upon whether FLAG is currently set to be 0 or 1, and then it inverts the value of FLAG by performing a bitwise XOR operation with 1.

As for the rest of the code (the init() function, the other variables being declared, and the sei() function), there is not enough context provided by the code to determine what exactly it is doing/trying to do.

This page might be helpful: http://www.avr-asm-tutorial.net/avr_en/beginner/PDETAIL.html

It appears your code is setting register values on an ATMEL AVR embedded processor.