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ATTINY87 Datasheet(PDF) 11 Page - ATMEL Corporation |
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ATTINY87 Datasheet(HTML) 11 Page - ATMEL Corporation |
11 / 282 page 11 7728D–AVR–07/09 ATtiny87/ATtiny167 Figure 2-3. The X-, Y-, and Z-registers In the different addressing modes these address registers have functions as fixed displace- ment, automatic increment, and automatic decrement (see the instruction set reference for details). 2.5 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 Register 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 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 0x60. 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 number of bits actually used is implementation dependent. Note that the data space in some implementa- tions of the AVR architecture is so small that only SPL is needed. In this case, the SPH Register will not be present 2.5.1 SPH and SPL – Stack Pointer Register 15 XH XL 0 X-register 7 0 7 0 R27 (0x1B) R26 (0x1A) 15 YH YL 0 Y-register 7 0 7 0 R29 (0x1D) R28 (0x1C) 15 ZH ZL 0 Z-register 7 0 7 0 R31 (0x1F) R30 (0x1E) Bit 15 141312 1110 9 8 SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL 7 654 321 0 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W Initial Value ISRAM end (See Table 3-1 on page 15) |
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