PIC16F627A_05 Datasheet, PDF (30/180 Pages) Microchip Technology – Flash-Based, 8-Bit CMOS Microcontrollers with nanoWatt Technology

PIC16F627A/628A/648A
4.3 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 high byte (PC<12:8>) is not
directly readable or writable and comes from PCLATH.
On any Reset, the PC is cleared. Figure 4-4 shows the
two situations for loading the PC. The upper example
in Figure 4-4 shows how the PC is loaded on a write to
PCL (PCLATH<4:0> → PCH). The lower example in
Figure 4-4 shows how the PC is loaded during a CALL
or GOTO instruction (PCLATH<4:3> → PCH).
FIGURE 4-4:
LOADING OF PC IN
DIFFERENT SITUATIONS
12
PC
PCH
PCL
87
PCLATH<4:0>
5
0
8
Instruction with
PCL as
Destination
ALU result
PCLATH
PCH
12 11 10
PC
87
2 ^PCLATH<4:3>
PCL
0
GOTO, CALL
11
Opcode <10:0>
PCLATH
4.3.1 COMPUTED GOTO
A computed GOTO is 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).
4.3.2 STACK
The PIC16F627A/628A/648A family has an 8-level
deep x 13-bit wide hardware stack (Figure 4-1). 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 CALL instruction
is executed or an interrupt causes a branch. The stack
is POPed in 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).
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.
4.4 Indirect Addressing, INDF and
FSR Registers
The INDF register is not a physical register. Addressing
the INDF register will cause indirect addressing.
Indirect addressing is possible by using the INDF
register. Any instruction using the INDF register
actually accesses data pointed to by the File Select
Register (FSR). Reading INDF itself indirectly will
produce 00h. Writing to the INDF register indirectly
results in a no-operation (although Status bits may be
affected). An effective 9-bit address is obtained by
concatenating the 8-bit FSR register and the IRP bit
(STATUS<7>), as shown in Figure 4-5.
A simple program to clear RAM location 20h-2Fh using
indirect addressing is shown in Example 4-1.
EXAMPLE 4-1: INDIRECT ADDRESSING
NEXT
MOVLW
MOVWF
CLRF
INCF
BTFSS
GOTO
0x20
FSR
INDF
FSR
FSR,4
NEXT
;initialize pointer
;to RAM
;clear INDF register
;inc pointer
;all done?
;no clear next
;yes continue
DS40044D-page 28
© 2005 Microchip Technology Inc.