Rainbow Electronics ATtiny26L - Manual (Page 34)

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ATtiny26(L)
1477B–AVR–04/02
Interrupt Handling The ATtiny26/L has two 8-bit Interrupt Mask Control Registers; GIMSK – General Inter-
rupt Mask Register and TIMSK – Timer/Counter Interrupt Mask Register.
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared (zero) and all inter-
rupts are disabled. The user software can set (one) the I-bit to enable nested interrupts.
The I-bit is set (one) when a Return from Interrupt instruction – RETI – is executed.
When the Program Counter is vectored to the actual Interrupt Vector in order to execute
the interrupt handling routine, hardware clears the corresponding flag that generated the
interrupt. Some of the interrupt flags can also be cleared by writing a logic one to the flag
bit position(s) to be cleared.
If an interrupt condition occurs when the corresponding interrupt enable bit is cleared
(zero), the interrupt flag will be set and remembered until the interrupt is enabled, or the
flag is cleared by software.
If one or more interrupt conditions occur when the Global Interrupt Enable bit is cleared
(zero), the corresponding interrupt flag(s) will be set and remembered until the Global
Interrupt Enable bit is set (one), and will be executed by order of priority.
Note that external level interrupt does not have a flag, and will only be remembered for
as long as the interrupt condition is active.
Note that the Status Register is not automatically stored when entering an interrupt rou-
tine and restored when returning from an interrupt routine. This must be handled by
software.
Interrupt Response Time The interrupt execution response for all the enabled AVR interrupts is four clock cycles
minimum. After the four clock cycles the program vector address for the actual interrupt
handling routine is executed. During this four clock cycle period, the Program Counter
(10 bits) is pushed onto the Stack. The vector is a relative jump to the interrupt routine,
and this jump takes two clock cycles. If an interrupt occurs during execution of a multi-
cycle instruction, this instruction is completed before the interrupt is served.
A return from an interrupt handling routine takes four clock cycles. During these four
clock cycles, the Program Counter (10 bits) is popped back from the Stack. When AVR
exits from an interrupt, it will always return to the main program and execute one more
instruction before any pending interrupt is served. Note that the Status Register – SREG
– is not handled by the AVR hardware, neither for interrupts nor for subroutines. For the
routines requiring a storage of the SREG, this must be performed by user software.
General Interrupt Mask
Register – GIMSK
• Bit 7 – Res: Reserved Bit
This bit is a reserved bit in the ATtiny26/L and always reads as zero.
• Bit 6 – INT0: External Interrupt Request 0 Enable
When the INT0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one),
the external pin interrupt is enabled. The Interrupt Sense Control0 bits 1/0 (ISC01 and
ISC00) in the MCU general Control Register (MCUCR) define whether the external
interrupt is activated on rising or falling edge, on pin change, or low level of the INT0 pin.
Activity on the pin will cause an interrupt request even if INT0 is configured as an output.
Bit 7 6 5 4 3 2 1 0
$3B ($5B) – INT0 PCIE1 PCIE0 – – – – GIMSK
Read/Write R R/W R/W R/W R R R R
Initial Value 0 0 0 0 0 0 0 0