ATMEGA161 datasheet(22/134 Pages) ATMEL

22 / 134 page

ATmega161(L)

Stack Pointer – SP

The ATmega161 Stack Pointer is implemented as two 8-bit registers in the I/O space locations $3E ($5E) and $3D ($5D).

As the ATmega161 supports up to 64KB memory, all 16 bits are used.

The Stack Pointer points to the data SRAM stack area where the Subroutine and Interrupt Stacks are located. This Stack

space in the data SRAM must be defined by the program before any subroutine calls are executed or interrupts are

enabled. The stack pointer must be set to point above $60. The Stack Pointer is decremented by one when data is pushed

onto the Stack with the PUSH instruction, and it is decremented by two when an address is pushed onto the Stack with

subroutine calls and interrupts. The Stack Pointer is incremented by one when data is popped from the Stack with the POP

instruction, and it is incremented by two when an address is popped from the Stack with return from subroutine RET or

return from interrupt RETI.

Reset and Interrupt Handling

The ATmega161 provides 20 different interrupt sources. These interrupts and the separate reset vector, each have a sep-

arate program vector in the program memory space. All interrupts are assigned individual enable bits which must be set

(one) together with the I-bit in the status register in order to enable the interrupt.

The lowest addresses in the program memory space are automatically defined as the Reset and Interrupt vectors. The

complete list of vectors is shown in Table 2. The list also determines the priority levels of the different interrupts. The lower

the address the higher is the priority level. RESET has the highest priority, and next is INT0 – the External Interrupt

Request 0 etc.

Bit

14131211

$3E ($5E)

SP15

SP14

SP13

SP12

SP11

SP10

SP9

SP8

SPH

$3D ($5D)

SP7

SP6

SP5

SP4

SP3

SP2

SP1

SP0

SPL

543

210

Read/Write

R/W

Initial value

000

Table 2. Reset and Interrupt Vectors

Vector No.

Program Address

Source

Interrupt Definition

$000

RESET

External Pin, Power-on Reset,

Brown-out Reset and Watchdog Reset

$002

INT0

External Interrupt Request 0

$004

INT1

External Interrupt Request 1

$006

INT2

External Interrupt Request 2

$008

TIMER2 COMP

Timer/Counter2 Compare Match

$00a

TIMER2 OVF

Timer/Counter2 Overflow

$00c

TIMER1 CAPT

Timer/Counter1 Capture Event

$00e

TIMER1 COMPA

Timer/Counter1 Compare Match A

$010

TIMER1 COMPB

Timer/Counter1 Compare Match B

$012

TIMER1 OVF

Timer/Counter1 Overflow

$014

TIMER0 COMP

Timer/Counter0 Compare Match

$016

TIMER0 OVF

Timer/Counter0 Overflow

$018

SPI, STC

Serial Transfer Complete

$01a

UART0, RX

UART0, Rx Complete

$01c

UART1, RX

UART1, Rx Complete

ATMEGA161 Datasheet(PDF) 22 Page - ATMEL Corporation

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