PIC16F716
FIGURE 2-3:
LOADING OF PC IN
DIFFERENT SITUATIONS
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 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 example in
Figure 2-3 shows how the PC is loaded during a CALLor
GOTOinstruction (PCLATH<4:3> → PCH).
PCH
12
PCL
8 7
0
Instruction with
PCL as
Destination
8
PCLATH<4:0>
ALU
5
PCLATH
PCL
PCH
12 1110
0
8 7
GOTO, CALL
11
PCLATH<4:3>
PCLATH
Opcode <10:0>
2.3.1
MODIFYING PCL
2
Executing any instruction with the PCL register as the
destination simultaneously causes the Program
Counter PC<12:8> bits (PCH) to be replaced by the
contents of the PCLATH register. This allows the entire
contents of the program counter to be changed by
writing the desired upper 5 bits to the PCLATH register.
When the lower 8 bits are written to the PCL register, all
13 bits of the program counter will change to the values
contained in the PCLATH register and those being
written to the PCL register.
2.4
Stack
The stack allows a combination of up to 8 program calls
and interrupts to occur. The stack contains the return
address from this branch in program execution.
Mid-range devices have 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 modified when the stack is PUSHed or
POPed.
A computed GOTOis accomplished by adding an offset
to the program counter (ADDWF PCL). Care should be
exercised when jumping into a look-up table or
program branch table (computed GOTO) by modifying
the PCL register. Assuming that PCLATH is set to the
table start address, if the table length is greater than
255 instructions or if the lower 8 bits of the memory
address rolls over from 0xFF to 0x00 in the middle of
the table, then PCLATH must be incremented for each
address rollover that occurs between the table
beginning and the target location within the table.
After the stack has been PUSHed 8 times, the ninth
push overwrites the value that was stored from the first
push. The tenth push overwrites the second push (and
so on).
For more information refer to Application Note AN556,
“Implementing a Table Read” (DS00556).
2.3.2
PROGRAM MEMORY PAGING
The CALL and GOTO instructions provide 11 bits of
address to allow branching within any 2K program
memory page. When doing a CALLor GOTOinstruction,
the upper bit of the address is provided by
PCLATH<3>. When doing a CALLor GOTOinstruction,
the user must ensure that the page select bit is
programmed so that the desired program memory
page is addressed. If a RETURNfrom a CALLinstruction
(or interrupt) is executed, the entire 13-bit PC is pushed
onto the stack. Therefore, manipulation of the
PCLATH<3> bit is not required for the RETURN
instructions (which POPs the address from the stack).
© 2007 Microchip Technology Inc.
DS41206B-page 17
MICROCHIP [ MICROCHIP ]
