Datasheet ATtiny28L, ATtiny28V (Atmel) - 6

Status Register Status Register – SREG
The AVR status register (SREG) at I/O space location $3F is defined as: Bit 7 6 5 4 3 2 1 0 $3F
I T H S V N Z C SREG
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0
• Bit 7 – I: Global Interrupt Enable
The global interrupt enable bit must be set (one) for the interrupts to be enabled. The individual interrupt enable control is then performed in separate control registers. If the global interrupt enable register is cleared (zero), none of the interrupts are enabled inde- pendent of the individual interrupt enable settings. The I-bit is cleared by hardware after an interrupt has occurred, and is set by the RETI instruction to enable subsequent interrupts.
• Bit 6 – T: Bit Copy Storage
The bit copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source and destination for the operated bit. A bit from a register in the register file can be copied into T by the BST instruction, and a bit in T can be copied into a bit in a register in the register file by the BLD instruction.
• Bit 5 – H: Half-carry Flag
The half-carry flag H indicates a half-carry in some arithmetic operations. See the Instruction Set description for detailed information.
• Bit 4 – S: Sign Bit, S = N
⊕
V
The S-bit is always an exclusive or between the negative flag N and the two’s comple- ment overflow flag V. See the Instruction Set description for detailed information.
• Bit 3 – V: Two’s Complement Overflow Flag
The two’s complement overflow flag V supports two’s complement arithmetic. See the Instruction Set description for detailed information.
• Bit 2 – N: Negative Flag
The negative flag N indicates a negative result from an arithmetical or logical operation. See the Instruction Set description for detailed information.
• Bit 1 – Z: Zero Flag
The zero flag Z indicates a zero result from an arithmetical or logical operation. See the Instruction Set description for detailed information.
• Bit 0 – C: Carry Flag
The carry flag C indicates a carry in an arithmetical or logical operation. See the Instruc- tion Set description for detailed information. 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.
6 ATtiny28L/V
1062F–AVR–07/06 Document Outline Features Pin Configurations Description Block Diagram Pin Descriptions VCC GND Port A (PA3..PA0) Port B (PB7..PB0) Port D (PD7..PD0) XTAL1 XTAL2 RESET Architectural Overview ALU - Arithmetic Logic Unit Subroutine and Interrupt Hardware Stack General-purpose Register File Status Register Status Register - SREG System Clock and Clock Options Internal RC Oscillator Calibrated Internal RC Oscillator Crystal Oscillator External Clock External RC Oscillator Register Description Oscillator Calibration Register - OSCCAL Memories I/O Memory Program and Data Addressing Modes Register Direct, Single Register Rd Register Indirect Register Direct, Two Registers Rd and Rr I/O Direct Relative Program Addressing, RJMP and RCALL Constant Addressing Using the LPM Instruction Memory Access and Instruction Execution Timing Flash Program Memory Sleep Modes Idle Mode Power-down Mode System Control and Reset Reset Sources Power-on Reset External Reset Watchdog Reset Register Description MCU Control and Status Register - MCUCS Interrupts Reset and Interrupt Interrupt Handling Interrupt Response Time External Interrupt Low-level Input Interrupt Register Description Interrupt Control Register - ICR Interrupt Flag Register - IFR I/O Ports Port A Port A as General Digital I/O Alternate Function of PA2 Port A Schematics Port B Port B as General Digital Input Alternate Functions of Port B Port B Schematics Port D Port D as General Digital I/O Register Description Port A Data Register - PORTA Port A Control Register - PACR Port A Input Pins Address - PINA Port B Input Pins Address - PINB Port D Data Register - PORTD Port D Data Direction Register - DDRD Port D Input Pins Address - PIND Timer/Counter0 Timer/Counter Prescaler Register Description Timer/Counter0 Control Register - TCCR0 Timer Counter 0 - TCNT0 Watchdog Timer Register Description Watchdog Timer Control Register - WDTCR Hardware Modulator Register Description Modulation Control Register - MODCR Analog Comparator Register Description Analog Comparator Control and Status Register - ACSR Memory Programming Program Memory Lock Bits Fuse Bits Signature Bytes Calibration Byte Programming the Flash Parallel Programming Signal Names Enter Programming Mode Chip Erase Programming the Flash Reading the Flash Programming the Fuse Bits Programming the Lock Bits Reading the Fuse and Lock Bits Reading the Signature Bytes and Calibration Byte Parallel Programming Characteristics Electrical Characteristics Absolute Maximum Ratings DC Characteristics External Clock Drive Waveforms External Clock Drive Typical Characteristics Register Summary Instruction Set Summary Ordering Information Packaging Information 32A 28P3 32M1-A Errata All revisions Datasheet Revision History Rev - 01/06G Rev - 01/06G Rev - 03/05F Table of Contents