PIC16F7X7
The stack operates as a circular buffer. This means that
after the stack has been PUSHed eight times, the ninth
push overwrites the value that was stored from the first
push. The tenth push overwrites the second push (and
so on).
2.3
PCL and PCLATH
The Program Counter (PC) is 13 bits wide. The low
byte comes from the PCL register which is a readable
and writable register. The upper bits (PC<12:8>) are
not readable but are indirectly writable through the
PCLATH register. On any Reset, the upper bits of the
PC will be cleared. Figure 2-4 shows the two situations
for the loading of the PC. The upper example in the
figure shows how the PC is loaded on a write to PCL
(PCLATH<4:0> → PCH). The lower example in the
figure shows how the PC is loaded during a CALL or
GOTOinstruction (PCLATH<4:3> → PCH).
Note 1: There are no Status bits to indicate stack
overflow or stack underflow conditions.
2: There are no instructions/mnemonics
called PUSH or POP. These are actions
that occur from the execution of the
CALL, RETURN, RETLW and RETFIE
instructions or the vectoring to an
interrupt address.
FIGURE 2-4:
LOADING OF PC IN
DIFFERENT SITUATIONS
2.4
Program Memory Paging
PIC16F7X7 devices are capable of addressing a con-
tinuous 8K word block of program memory. The CALL
and GOTOinstructions provide only 11 bits of address to
allow branching within any 2K program memory page.
When doing a CALL or GOTO instruction, the upper
2 bits of the address are provided by PCLATH<4:3>.
When doing a CALLor GOTOinstruction, the user must
ensure that the page select bits are programmed so
that the desired program memory page is addressed. If
a return from a CALL instruction (or interrupt) is
executed, the entire 13-bit PC is POPed off the stack.
Therefore, manipulation of the PCLATH<4:3> bits is
not required for the RETURNinstructions (which POPs
the address from the stack).
PCH
PCL
12
8
7
0
Instruction with
PCL as
Destination
PC
8
PCLATH<4:0>
PCLATH
5
ALU
PCH
12 11 10
PC
PCL
8
7
0
GOTO,CALL
PCLATH<4:3>
PCLATH
11
2
Opcode <10:0>
Note:
The contents of the PCLATH are
unchanged after a RETURN or RETFIE
instruction is executed. The user must set
up the PCLATH for any subsequent CALLs
or GOTOs.
2.3.1
COMPUTED GOTO
A computed GOTOis accomplished by adding an offset
to the program counter (ADDWF PCL). When doing a
table read using a computed GOTO method, care
should be exercised if the table location crosses a PCL
memory boundary (each 256-byte block). Refer to the
Application Note, AN556 “Implementing a Table Read”
(DS00556).
Example 2-1 shows the calling of a subroutine in
page 1 of the program memory. This example assumes
that PCLATH is saved and restored by the Interrupt
Service Routine (if interrupts are used).
EXAMPLE 2-1:
CALL OF A SUBROUTINE
IN PAGE 1 FROM PAGE 0
2.3.2
STACK
ORG
BCF
BSF
0x500
PCLATH, 4
PCLATH, 3 ;Select page 1
;(800h-FFFh)
The PIC16F7X7 family has an 8-level deep x 13-bit
wide hardware stack. The stack space is not part of
either program or data space and the stack pointer is
not readable or writable. The PC is PUSHed onto the
stack when a CALL instruction is executed or an
interrupt causes a branch. The stack is POPed in the
event of a RETURN, RETLWor a RETFIE instruction
execution. PCLATH is not affected by a PUSH or POP
operation.
CALL SUB1_P1
:
:
ORG
;Call subroutine in
;page 1 (800h-FFFh)
0x900
;page 1 (800h-FFFh)
SUB1_P1
RETURN
:
:
:
;called subroutine
;page 1 (800h-FFFh)
;return to Call
;subroutine in page 0
;(000h-7FFh)
2004 Microchip Technology Inc.
DS30498C-page 29
MICROCHIP [ MICROCHIP ]
