ST7LITE02 datasheet(22/122 Pages) STMICROELECTRONICS

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ST7LITE0, ST7SUPERLITE

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CPU REGISTERS (Cont’d)

Stack Pointer (SP)

Read/Write

Reset Value: 00 FFh

The Stack Pointer is a 16-bit register which is al-

ways pointing to the next free location in the stack.

It is then decremented after data has been pushed

onto the stack and incremented before data is

popped from the stack (see Figure 11).

Since the stack is 64 bytes deep, the 10 most sig-

nificant bits are forced by hardware. Following an

MCU Reset, or after a Reset Stack Pointer instruc-

tion (RSP), the Stack Pointer contains its reset val-

ue (the SP5 to SP0 bits are set) which is the stack

higher address.

The least significant byte of the Stack Pointer

(called S) can be directly accessed by a LD in-

struction.

Note: When the lower limit is exceeded, the Stack

Pointer wraps around to the stack upper limit, with-

out indicating the stack overflow. The previously

stored information is then overwritten and there-

fore lost. The stack also wraps in case of an under-

flow.

The stack is used to save the return address dur-

ing a subroutine call and the CPU context during

an interrupt. The user may also directly manipulate

the stack by means of the PUSH and POP instruc-

tions. In the case of an interrupt, the PCL is stored

at the first location pointed to by the SP. Then the

other registers are stored in the next locations as

shown in Figure 11.

– When an interrupt is received, the SP is decre-

mented and the context is pushed on the stack.

– On return from interrupt, the SP is incremented

and the context is popped from the stack.

A subroutine call occupies two locations and an in-

terrupt five locations in the stack area.

Figure 11. Stack Manipulation Example

000

SP5

SP4

SP3

SP2

SP1

SP0

PCH

PCL

PCH

PCL

PCH

PCL

PCH

PCL

PCH

PCL

CALL

Subroutine

Interrupt

event

PUSH Y

POP Y

IRET

RET

or RSP

@ 00FFh

@ 00C0h

Stack Higher Address = 00FFh

Stack Lower Address = 00C0h

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