Explain about code timer0, help me !

[iVoVa]

[Ttelmah]

OK.
Think of the timer, like a sort of clock face. It is numbered from 0 (at the top), round to 65535.
Behind it is a gearbox, so it takes 65536 turns of the input shaft to go round once.
Then behind this is another gearbox, with a number of output shafts fed from the motor. The motor goes through a 4:1 reduction, then feeds the first shaft. There are four output shafts. One at the motor speed/4, one at half this, one at 1/4 this, and one at one eighth this.
Then there is one final thing. At the top of the face, is a bell. This rings when the hand goes from 65535, to 0.

When you start, the motor is spinning, but none of the four shafts connect to the main 'face' gearbox. Then:
setup_timer_0(RTCC_INTERNAL|RTCC_DIV_2);

Connects the shaft at motor/8 to the face gearbox.
Then:
set_timer0(0x23);

Moves the hand forwards to '35'.

The hand now moves, and counts round to the top, where the bell rings.

enable_interrupts(global);
enable_interrupts(int_timer0);

says 'when the bell rings, execute the int_timer0 code'.

At no point is the gearbox disconnected (setup_timer_0(RTCC_OFF)), or the bell stopped (disable_interrupts(INT_TIMER0)), so the bell keeps ringing at regular intervals (close to one second).

The timing will not be perfect, since the hand moves forwards a little, between the bell going off (timer interrupt triggering), and the code actually reaching the instruction to move the hand forward a little in the interrupt code. These counts will be 'extra', So the code will take this little amount extra each second, and will run about 0.1% slow.

The point is that once started, the hand keeps going round, and so the interrupt code will keep being called.

Best Wishes

[iVoVa]

@Ttelmah : Thanks for helping !