20
AT90S/LS4433
1042G–AVR–09/02
Stack Pointer – SP The AT90S4433 Stack Pointer is implemented as an 8-bit register in the I/O space loca-
tion $3D ($5D). As the AT90S4433 data memory has $0DF locations, eight 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 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 AT90S4433 provides 13 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, 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 3. 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.
76543210
$3D ($5D) SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SP
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
InitialValue00000000
Table 3. Reset and Interrupt Vectors
Vector No.
Program
Address Source Interrupt Definition
1 $000 RESET
External Pin, Power-on Reset, Brown-out Reset
and Watchdog Reset
2 $001 INT0 External Interrupt Request 0
3 $002 INT1 External Interrupt Request 1
4 $003 TIMER1 CAPT Timer/Counter1 Capture Event
5 $004 TIMER1 COMP Timer/Counter1 Compare Match
6 $005 TIMER1 OVF Timer/Counter1 Overflow
7 $006 TIMER0 OVF Timer/Counter0 Overflow
8 $007 SPI, STC Serial Transfer Complete
9 $008 UART, RX UART, Rx Complete
10 $009 UART, UDRE UART Data Register Empty
11 $00A UART, TX UART, Tx Complete
12 $00B ADC ADC Conversion Complete
13 $00C EE_RDY EEPROM Ready
14 $00D ANA_COMP Analog Comparator
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