Microchip Technology DV164136 Data Sheet
PIC18F8722 FAMILY
DS39646C-page 68
© 2008 Microchip Technology Inc.
5.1.3.4
Stack Full and Underflow Resets
Device Resets on stack overflow and stack underflow
conditions are enabled by setting the STVREN bit in
Configuration Register 4L. When STVREN is set, a full
or underflow will set the appropriate STKFUL or
STKUNF bit and then cause a device Reset. When
STVREN is cleared, a full or underflow condition will set
the appropriate STKFUL or STKUNF bit, but not cause
a device Reset. The STKFUL or STKUNF bits are
cleared by the user software or a Power-on Reset.
conditions are enabled by setting the STVREN bit in
Configuration Register 4L. When STVREN is set, a full
or underflow will set the appropriate STKFUL or
STKUNF bit and then cause a device Reset. When
STVREN is cleared, a full or underflow condition will set
the appropriate STKFUL or STKUNF bit, but not cause
a device Reset. The STKFUL or STKUNF bits are
cleared by the user software or a Power-on Reset.
5.1.4
FAST REGISTER STACK
A fast register stack is provided for the STATUS,
WREG and BSR registers, to provide a “fast return”
option for interrupts. The stack for each register is only
one level deep and is neither readable nor writable. It is
loaded with the current value of the corresponding reg-
ister when the processor vectors for an interrupt. All
interrupt sources will push values into the Stack regis-
ters. The values in the registers are then loaded back
into their associated registers if the RETFIE, FAST
instruction is used to return from the interrupt.
WREG and BSR registers, to provide a “fast return”
option for interrupts. The stack for each register is only
one level deep and is neither readable nor writable. It is
loaded with the current value of the corresponding reg-
ister when the processor vectors for an interrupt. All
interrupt sources will push values into the Stack regis-
ters. The values in the registers are then loaded back
into their associated registers if the RETFIE, FAST
instruction is used to return from the interrupt.
If both low and high-priority interrupts are enabled, the
stack registers cannot be used reliably to return from
low-priority interrupts. If a high-priority interrupt occurs
while servicing a low-priority interrupt, the Stack regis-
ter values stored by the low-priority interrupt will be
overwritten. In these cases, users must save the key
registers in software during a low-priority interrupt.
stack registers cannot be used reliably to return from
low-priority interrupts. If a high-priority interrupt occurs
while servicing a low-priority interrupt, the Stack regis-
ter values stored by the low-priority interrupt will be
overwritten. In these cases, users must save the key
registers in software during a low-priority interrupt.
If interrupt priority is not used, all interrupts may use the
fast register stack for returns from interrupt. If no inter-
rupts are used, the fast register stack can be used to
restore the STATUS, WREG and BSR registers at the
end of a subroutine call. To use the fast register stack
for a subroutine call, a CALL label, FAST instruction
must be executed to save the STATUS, WREG and
BSR registers to the fast register stack. A
RETURN
fast register stack for returns from interrupt. If no inter-
rupts are used, the fast register stack can be used to
restore the STATUS, WREG and BSR registers at the
end of a subroutine call. To use the fast register stack
for a subroutine call, a CALL label, FAST instruction
must be executed to save the STATUS, WREG and
BSR registers to the fast register stack. A
RETURN
, FAST instruction is then executed to restore
these registers from the fast register stack.
Example 5-1 shows a source code example that uses
the fast register stack during a subroutine call and return.
the fast register stack during a subroutine call and return.
EXAMPLE 5-1:
FAST REGISTER STACK
CODE EXAMPLE
CODE EXAMPLE
5.1.5
LOOK-UP TABLES IN PROGRAM
MEMORY
MEMORY
There may be programming situations that require the
creation of data structures, or look-up tables, in
program memory. For PIC18 devices, look-up tables
can be implemented in two ways:
creation of data structures, or look-up tables, in
program memory. For PIC18 devices, look-up tables
can be implemented in two ways:
• Computed GOTO
• Table Reads
• Table Reads
5.1.5.1
Computed GOTO
A computed GOTO is accomplished by adding an offset
to the program counter. An example is shown in
Example 5-2.
to the program counter. An example is shown in
Example 5-2.
A look-up table can be formed with an ADDWF PCL
instruction and a group of RETLW nn instructions. The W
register is loaded with an offset into the table before exe-
cuting a call to that table. The first instruction of the called
routine is the ADDWF PCL instruction. The next instruction
executed will be one of the RETLW nn instructions that
returns the value ‘nn’ to the calling function.
instruction and a group of RETLW nn instructions. The W
register is loaded with an offset into the table before exe-
cuting a call to that table. The first instruction of the called
routine is the ADDWF PCL instruction. The next instruction
executed will be one of the RETLW nn instructions that
returns the value ‘nn’ to the calling function.
The offset value (in WREG) specifies the number of
bytes that the program counter should advance and
should be multiples of 2 (LSb = 0).
bytes that the program counter should advance and
should be multiples of 2 (LSb = 0).
In this method, only one data byte may be stored in
each instruction location and room on the return
address stack is required.
each instruction location and room on the return
address stack is required.
EXAMPLE 5-2:
COMPUTED GOTO USING AN OFFSET VALUE
Note:
The “ADDWF PCL” instruction does not
update the PCLATH and PCLATU registers.
A read operation on PCL must be performed
to update PCLATH and PCLATU.
update the PCLATH and PCLATU registers.
A read operation on PCL must be performed
to update PCLATH and PCLATU.
CALL SUB1, FAST
;STATUS, WREG, BSR
;SAVED IN FAST REGISTER
;STACK
•
•
•
SUB1
•
•
•
RETURN, FAST
;RESTORE VALUES SAVED
;IN FAST REGISTER STACK
MAIN:
ORG
0x0000
MOVLW
0x00
CALL
TABLE
…
ORG
0x8000
TABLE
MOVF
PCL, F
; A simple read of PCL will update PCLATH, PCLATU
RLNCF
W, W
; Multiply by 2 to get correct offset in table
ADDWF
PCL
; Add the modified offset to force jump into table
RETLW
‘A’
RETLW
‘B’
RETLW
‘C’
RETLW
‘D’
RETLW
‘E’
END