PIC18F2331 Datasheet, PDF (61/396 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
REGISTER 5-1: STKPTR REGISTER
R/C-0
R/C-0
U-0
STKFUL STKUNF
—
bit 7
R/W-0
SP4
bit 7⁽¹⁾
bit 6⁽¹⁾
bit 5
bit 4-0
STKFUL: Stack Full Flag bit
1 = Stack became full or overflowed
0 = Stack has not become full or overflowed
STKUNF: Stack Underflow Flag bit
1 = Stack underflow occurred
0 = Stack underflow did not occur
Unimplemented: Read as ‘0’
SP4:SP0: Stack Pointer Location bits
R/W-0
SP3
R/W-0
SP2
R/W-0
SP1
R/W-0
SP0
bit 0
Note 1: Bit 7 and bit 6 are cleared by user software or by a POR.
Legend:
R = Readable bit
- n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented C = Clearable only bit
‘0’ = Bit is cleared
x = Bit is unknown
5.2.3 PUSH AND POP INSTRUCTIONS
Since the Top-of-Stack (TOS) 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 option. To push the current PC value
onto the stack, a PUSH instruction can be executed.
This will increment the stack pointer and load the cur-
rent PC value onto the stack. TOSU, TOSH and TOSL
can then be modified to place data or a return address
on the stack.
The ability to pull the TOS value off of the stack and
replace it with the value that was previously pushed
onto the stack, without disturbing normal execution, is
achieved by using the POP instruction. The POP instruc-
tion discards the current TOS by decrementing the
stack pointer. The previous value pushed onto the
stack then becomes the TOS value.
5.2.4 STACK FULL/UNDERFLOW RESETS
These Resets are enabled by programming the
STVREN bit in Configuration Register 4L. When the
STVREN bit is cleared, a full or underflow condition will
set the appropriate STKFUL or STKUNF bit, but not
cause a device Reset. When the STVREN bit is set, a
full or underflow will set the appropriate STKFUL or
STKUNF bit and then cause a device Reset. The
STKFUL or STKUNF bits are cleared by the user
software or a POR Reset.
 2003 Microchip Technology Inc.
Preliminary
DS39616B-page 59