at90pwm81-16se ATMEL Corporation, at90pwm81-16se Datasheet - Page 10

• Manufacturer Part Number: at90pwm81-16se
• Description: 8-bit Avr Microcontroller With 8k Bytes In- System Programmable Flash
• Manufacturer: ATMEL Corporation

10
AT90PWM81
Note that the Status Register is updated after all ALU operations, as specified in the Instruction Set Refer-
ence. This will in many cases remove the need for using the dedicated compare instructions, resulting in
faster and more compact code.
The Status Register is not automatically stored when entering an interrupt routine and restored when
returning from an interrupt. This must be handled by software.
The AVR Status Register – SREG – is defined as:
• Bit 7 – I: Global Interrupt Enable
The Global Interrupt Enable bit must be set to enabled the interrupts. The individual interrupt enable con-
trol is then performed in separate control registers. If the Global Interrupt Enable Register is cleared, none
of the interrupts are enabled independent 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 inter-
rupts. The I-bit can also be set and cleared by the application with the SEI and CLI instructions, as
described in the instruction set reference.
• Bit 6 – T: Bit Copy Storage
The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or 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. Half Carry Is useful in BCD
arithmetic. See the “Instruction Set Description” for detailed information.
• Bit 4 – S: Sign Bit, S = N
The S-bit is always an exclusive or between the negative flag N and the Two’s Complement 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 in an arithmetic or logic operation. See the “Instruction
Set Description” for detailed information.
• Bit 1 – Z: Zero Flag
The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the “Instruction Set
Description” for detailed information.
• Bit 0 – C: Carry Flag
The Carry Flag C indicates a carry in an arithmetic or logic operation. See the “Instruction Set Descrip-
tion” for detailed information.
Bit
Read/Write
Initial Value
7
I
R/W
0
6
T
R/W
0
⊕
5
H
R/W
0
V
4
S
R/W
0
3
V
R/W
0
2
N
R/W
0
1
Z
R/W
0
0
C
R/W
0
SREG
7734M–AVR–03/10