AT90LS8535-4AI datasheet(20/127 Pages) ATMEL

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AT90S/LS8535

1041H–11/01

Stack Pointer – SP

The AT90S8535 Stack Pointer is implemented as two 8-bit registers in the I/O space

locations $3E ($5E) and $3D ($5D). As the AT90S8535 data memory has $25F loca-

tions, 10 bits are used.

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 1 when

data is pushed onto the stack with the PUSH instruction and it is decremented by 2

when an address is pushed onto the stack with subroutine calls and interrupts. The

Stack Pointer is incremented by 1 when data is popped from the stack with the POP

instruction and it is incremented by 2 when an address is popped from the stack with

return from subroutine RET or return from interrupt RETI.

Reset and Interrupt

Handling

The AT90S8535 provides 16 different interrupt sources. These interrupts and the sepa-

rate reset vector each have a separate program vector in the program memory space.

All interrupts are assigned individual enable bits that 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 the priority level. RESET has the highest priority, and next is INT0 (the External

Interrupt Request 0), etc.

Bit

1514

1312

1110

$3E ($5E)

––

––––

SP9

SP8

SPH

$3D ($5D)

SP7

SP6

SP5

SP4

SP3

SP2

SP1

SP0

SPL

543210

Read/Write

RRRRR

R/W

Initial Value

000000

Table 2. Reset and Interrupt Vectors

Vector No.

Program Address

Source

Interrupt Definition

$000

RESET

Hardware Pin, Power-on Reset and

Watchdog Reset

$001

INT0

External Interrupt Request 0

$002

INT1

External Interrupt Request 1

$003

TIMER2 COMP

Timer/Counter2 Compare Match

$004

TIMER2 OVF

Timer/Counter2 Overflow

$005

TIMER1 CAPT

Timer/Counter1 Capture Event

$006

TIMER1 COMPA

Timer/Counter1 Compare Match A

$007

TIMER1 COMPB

Timer/Counter1 Compare Match B

$008

TIMER1 OVF

Timer/Counter1 Overflow

$009

TIMER0 OVF

Timer/Counter0 Overflow

$00A

SPI, STC

SPI Serial Transfer Complete

$00B

UART, RX

UART, Rx Complete

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