I'm trying to learn about interrupts and registers on atmega microcontrollers and I'm guiding my self with http://www.gammon.com.au/forum/bbshowpost.php?id=11488 from your friend Nick Gammon which had help me giving my first steps understanding something.
Today I found a register on an example Nick give there and I don´t understand it - SREG
Can someone explain me what this register is for?
SREG is the processor Status REGister. The interesting bit from the standpoint of interrupts is Bit 7, the Global Interrupt Enable bit. It is cleared when you enter an ISR to prevent other interrupts from interrupting the ISR. You can set it when you are sure that another interrupt won't cause problems or you can let it be set automatically when you return from the ISR.
So in order for I use interrupts I need to set this bit and after trigger an interrupt set it again to be ready for use again?
HugoPT:
So in order for I use interrupts I need to set this bit and after trigger an interrupt set it again to be ready for use again?
When your sketch starts the bit is ON.
If you turn the bit OFF you will get no interrupts until you turn the bit ON again.
You can turn the bit ON and OFF by writing to SREG or using the 'cli' (Clear Interrupt Enable) and 'sei' (Set Interrupt Enable) instructions.
HugoPT:
So in order for I use interrupts I need to set this bit and after trigger an interrupt set it again to be ready for use again?
You don't normally need to use SREG at all. Nor do you need to turn interrupts on and off like you describe.
Have another read of my page. Interrupts start "on". They are turned off inside an interrupt service routine, automatically, and turn back on afterwards, automatically.
Unless you are doing something pretty specific, like making sure interrupts are off during atomic access to a variable, as described near the end of my page about interrupts, you don't need to touch SREG.