PIC16C54C-04/SO【PDF 148/194 页】IC MCU OTP 512X12 18SOIC

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PIC18F2450/4450

2006 Microchip Technology Inc.

Advance Information

DS39760A-page 55

5.1.2.2

Return Stack Pointer (STKPTR)

The STKPTR register (Register 5-1) contains the Stack

Pointer value, the STKFUL (Stack Full) status bit and

the STKUNF (Stack Underflow) status bit. The value of

the Stack Pointer can be 0 through 31. The Stack

Pointer increments before values are pushed onto the

stack and decrements after values are popped off the

stack. On Reset, the Stack Pointer value will be zero.

The user may read and write the Stack Pointer value.

This feature can be used by a Real-Time Operating

System (RTOS) for return stack maintenance.

After the PC is pushed onto the stack 31 times (without

popping any values off the stack), the STKFUL bit is

set. The STKFUL bit is cleared by software or by a

POR.

The action that takes place when the stack becomes

full depends on the state of the STVREN (Stack

Overflow Reset Enable) Configuration bit. (Refer to

Section 18.1 “Configuration Bits” for a description of

the device Configuration bits.) If STVREN is set

(default), the 31st push will push the (PC + 2) value

onto the stack, set the STKFUL bit and reset the

device. The STKFUL bit will remain set and the Stack

Pointer will be set to zero.

If STVREN is cleared, the STKFUL bit will be set on the

31st push and the Stack Pointer will increment to 31.

Any additional pushes will not overwrite the 31st push

and the STKPTR will remain at 31.

When the stack has been popped enough times to

unload the stack, the next pop will return a value of zero

to the PC and sets the STKUNF bit, while the Stack

Pointer remains at zero. The STKUNF bit will remain

set until cleared by software or until a POR occurs.

5.1.2.3

PUSH

and POP Instructions

Since the Top-of-Stack is readable and writable, the

ability to push values onto the stack and pull values off

the stack, without disturbing normal program execu-

tion, is a desirable feature. The PIC18 instruction set

includes two instructions, PUSH and POP, that permit

the TOS to be manipulated under software control.

TOSU, TOSH and TOSL can be modified to place data

or a return address on the stack.

The PUSH instruction places the current PC value onto

the stack. This increments the Stack Pointer and loads

the current PC value onto the stack.

The POP instruction discards the current TOS by

decrementing the Stack Pointer. The previous value

pushed onto the stack then becomes the TOS value.

Note:

Returning a value of zero to the PC on an

underflow has the effect of vectoring the

program to the Reset vector, where the

stack conditions can be verified and

appropriate actions can be taken. This is

not the same as a Reset, as the contents

of the SFRs are not affected.

REGISTER 5-1:

STKPTR: STACK POINTER REGISTER

R/C-0

U-0

R/W-0

STKFUL(1)

STKUNF(1)

—

SP4

SP3

SP2

SP1

SP0

bit 7

bit 0

Legend:

C = Clearable bit

R = Readable bit

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

bit 7

STKFUL: Stack Full Flag bit(1)

= Stack became full or overflowed

= Stack has not become full or overflowed

bit 6

STKUNF: Stack Underflow Flag bit(1)

= Stack underflow occurred

= Stack underflow did not occur

bit 5

Unimplemented: Read as ‘0’

bit 4-0

SP4:SP0: Stack Pointer Location bits

Note 1:

Bit 7 and bit 6 are cleared by user software or by a POR.

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