Datasheet

9
ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
Program flow is provided by conditional and unconditional jump and call instructions, able to directly address the whole
address space. Most AVR
®
instructions have a single 16-bit word format. Every program memory address contains a 16- or
32-bit instruction.
Program flash memory space is divided in two sections, the boot program section and the application program section. Both
sections have dedicated lock bits for write and read/write protection. The SPM instruction that writes into the application flash
memory section must reside in the boot program section.
During interrupts and subroutine calls, the return address program counter (PC) is stored on the stack. The stack is
effectively allocated in the general data SRAM, and consequently the stack size is only limited by the total SRAM size and
the usage of the SRAM. All user programs must initialize the SP in the reset routine (before subroutines or interrupts are
executed). The stack pointer (SP) is read/write accessible in the I/O space. The data SRAM can easily be accessed through
the five different addressing modes supported in the AVR architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional global interrupt enable bit in the status
register. All interrupts have a separate interrupt vector in the interrupt vector table. The interrupts have priority in accordance
with their interrupt vector position. The lower the interrupt vector address, the higher the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as control registers, SPI, and other I/O functions.
The I/O memory can be accessed directly, or as the data space locations following those of the register file, 0x20 - 0x5F. In
addition, the Atmel
®
ATmega48PA/88PA/168PA has extended I/O space from 0x60 - 0xFF in SRAM where only the
ST/STS/STD and LD/LDS/LDD instructions can be used.
7.2 ALU – Arithmetic Logic Unit
The high-performance AVR ALU operates in direct connection with all the 32 general purpose working registers. Within a
single clock cycle, arithmetic operations between general purpose registers or between a register and an immediate are
executed. The ALU operations are divided into three main categories – arithmetic, logical, and bit-functions. Some
implementations of the architecture also provide a powerful multiplier supporting both signed/unsigned multiplication and
fractional format. See Section 32. “Instruction Set Summary” on page 317 for a detailed description.
7.3 Status Register
The status register contains information about the result of the most recently executed arithmetic instruction. This
information can be used for altering program flow in order to perform conditional operations. Note that the status register is
updated after all ALU operations, as specified in the instruction set reference. 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.
7.3.1 SREG – AVR Status Register
The AVR status register – SREG – is defined as:
• Bit 7 – I: Global Interrupt Enable
The global interrupt enable bit must be set 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, 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 interrupts. 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 76543210
0x3F (0x5F) 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