PIC18FXX39 Datasheet, PDF (35/322 Pages) Microchip Technology – Enhanced FLASH Microcontrollers with Single Phase Induction Motor Control Kernel

PIC18FXX39
REGISTER 4-1: 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
R/W-0
SP0
bit 0
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 can only be cleared in user software or by a POR.
Legend:
R = Readable bit
- n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared x = Bit is unknown
FIGURE 4-2:
RETURN ADDRESS STACK AND ASSOCIATED REGISTERS
TOSU
0x00
TOSH
0x1A
Return Address Stack
TOSL
0x34
11111
11110
11101
STKPTR<4:0>
00010
00011
Top-of-Stack 0x001A34 00010
0x000D58 00001
00000
4.2.3 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 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.
4.2.4 STACK FULL/UNDERFLOW RESETS
These RESETS are enabled by programming the
STVREN configuration bit. When the STVREN bit is
disabled, a full or underflow condition will set the appro-
priate STKFUL or STKUNF bit, but not cause a device
RESET. When the STVREN bit is enabled, 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 only cleared by the user
software or a POR Reset.
 2002-2013 Microchip Technology Inc.
Preliminary
DS30485B-page 35