PIC16F818/819
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 2-5 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 2-5:
LOADING OF PC IN
DIFFERENT SITUATIONS
2.4
Indirect Addressing: INDF and
FSR Registers
PCH
PCL
The INDF register is not a physical register. Addressing
INDF actually addresses the register whose address is
contained in the FSR register (FSR is a pointer). This is
indirect addressing.
12
8
7
0
Instruction with
PCL as
Destination
PC
8
PCLATH<4:0>
PCLATH
5
ALU
EXAMPLE 2-1:
INDIRECT ADDRESSING
• Register file 05 contains the value 10h
• Register file 06 contains the value 0Ah
• Load the value 05 into the FSR register
PCH
12 11 10
PC
PCL
8
7
0
GOTO,CALL
• A read of the INDF register will return the value
of 10h
PCLATH<4:3>
PCLATH
11
2
Opcode <10:0>
• Increment the value of the FSR register by one
(FSR = 06)
• A read of the INDF register now will return the
value of 0Ah
2.3.1
COMPUTED GOTO
Reading INDF itself indirectly (FSR = 0) will produce
00h. Writing to the INDF register indirectly results in a
no operation (although status bits may be affected).
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).
A simple program to clear RAM locations, 20h-2Fh,
using indirect addressing is shown in Example 2-2.
EXAMPLE 2-2:
HOW TO CLEAR RAM
USING INDIRECT
ADDRESSING
2.3.2
STACK
MOVLW 0x20
MOVWF FSR
CLRF INDF
INCF FSR
;initialize pointer
;to RAM
;clear INDF register
;inc pointer
The PIC16F818/819 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, RETLW or a RETFIE instruction
execution. PCLATH is not affected by a PUSH or POP
operation.
NEXT
BTFSS FSR, 4 ;all done?
GOTO
NEXT
;NO, clear next
;YES, continue
CONTINUE
:
An effective 9-bit address is obtained by concatenating
the 8-bit FSR register and the IRP bit (Status<7>) as
shown in Figure 2-6.
2004 Microchip Technology Inc.
DS39598E-page 23
MICROCHIP [ MICROCHIP ]
