15
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
3.6
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 0x100. 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 implementations of the AVR architecture is so small that only SPL
is needed. In this case, the SPH Register will not be present.
3.7
Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR CPU is driven by the CPU clock
clkCPU, directly generated from the selected clock source for the chip. No internal clock division is used.
Figure 3-4 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture and the fast-
access Register File concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz with the corresponding unique
results for functions per cost, functions per clocks, and functions per power-unit.
Figure 3-4.
The Parallel Instruction Fetches and Instruction Executions
Bit
15141312
11
10
9
8
SP15
SP14
SP13
SP12
SP11
SP10
SP9
SP8
SPH
SP7
SP6
SP5
SP4
SP3
SP2
SP1
SP0
SPL
76543210
Read/Write
R/W
Initial Value
Top address of the SRAM (0x04FF/0x08FF/0x10FF)
clk
CPU
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
T1
T2
T3
T4
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch