PIC16F688
2.3.2
STACK
2.3
PCL and PCLATH
The PIC16F688 family has an 8-level x 13-bit wide
hardware stack (see Figure 2-1). 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 CALLinstruction is executed or
an interrupt causes a branch. The stack is POPed in
The Program Counter (PC) is 13 bits wide. The low byte
comes from the PCL register, which is a readable and
writable register. The high byte (PC<12:8>) is not
directly readable or writable and comes from PCLATH.
On any Reset, the PC is cleared. Figure 2-3 shows the
two situations for the loading of the PC. The upper
example in Figure 2-3 shows how the PC is loaded on a
write to PCL (PCLATH<4:0> → PCH). The lower exam-
ple in Figure 2-3 shows how the PC is loaded during a
CALLor GOTOinstruction (PCLATH<4:3> → PCH).
the event of a RETURN,
RETLW or a RETFIE
instruction execution. PCLATH is not affected by a
PUSH or POP operation.
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).
FIGURE 2-3:
LOADING OF PC IN
DIFFERENT SITUATIONS
PCH
PCL
Instruction with
Note 1: There are no Status bits to indicate stack
12
8
7
0
PCL as
overflow or stack underflow conditions.
Destination
PC
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.
8
PCLATH<4:0>
PCLATH
5
ALU Result
PCH
12 11 10
PC
PCL
8
7
0
GOTO, CALL
PCLATH<4:3>
PCLATH
11
2
OPCODE<10:0>
2.3.1
COMPUTED GOTO
A computed GOTOis accomplished by adding an offset
to the program counter (ADDWF PCL). When perform-
ing a table read using a computed GOTOmethod, 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).
© 2007 Microchip Technology Inc.
DS41203D-page 19
MICROCHIP [ MICROCHIP ]
