PIC18F2220 Datasheet, PDF (57/388 Pages) Microchip Technology – 28/40/44-Pin High-Performance, Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology

PIC18F2220/2320/4220/4320
REGISTER 5-1: STKPTR REGISTER
R/C-0
R/C-0
U-0
STKFUL STKUNF
—
bit 7
R/W-0
SP4
R/W-0
SP3
R/W-0
SP2
R/W-0
SP1
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
Note 1: Bit 7 and bit 6 are cleared by user software or by a POR.
R/W-0
SP0
bit 0
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 condition 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.
DS39599C-page 55