ATmega16 datasheet(10/315 Pages) ATMEL

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ATmega16(L)

2466E–AVR–10/02

Stack Pointer

The Stack is mainly used for storing temporary data, for storing local variables and for

storing return addresses after interrupts and subroutine calls. The Stack Pointer Regis-

ter always points to the top of the stack. Note that the stack is implemented as growing

from higher memory locations to lower memory locations. This implies that a stack

PUSH command decreases the Stack Pointer.

The Stack Pointer points to the data SRAM stack area where the Subroutine and Inter-

rupt 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 the return address is pushed onto the Stack with subroutine call or interrupt. 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 data is popped from the Stack with return

from subroutine RET or return from interrupt RETI.

The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The num-

ber of bits actually used is implementation dependent. Note that the data space in some

implementations of the AVR architecture is so small that only SPL is needed. In this

case, the SPH Register will not be present.

Bit

1514

1312

1110

SP15

SP14

SP13

SP12

SP11

SP10

SP9

SP8

SPH

SP7

SP6

SP5

SP4

SP3

SP2

SP1

SP0

SPL

543210

Read/Write

R/W

Initial Value

000000

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