PIC16F193X Datasheet, PDF (54/418 Pages) Microchip Technology – 28/40/44-Pin Flash-Based, 8-Bit CMOS Microcontrollers with LCD Driver and nanoWatt Technology

PIC16F193X/LF193X
2.3 PCL and PCLATH
The Program Counter (PC) is 15 bits wide. The low byte
comes from the PCL register, which is a readable and
writable register. The high byte (PC<14:8>) is not directly
readable or writable and comes from PCLATH. On any
Reset, the PC is cleared. Figure 2-4 shows the five
situations for the loading of the PC.
FIGURE 2-4:
LOADING OF PC IN
DIFFERENT SITUATIONS
14 PCH
PC
6⁷
PCLATH
PCL
0 Instruction with
PCL as
Destination
0
8
ALU Result
14 PCH
PC
6⁴
PCLATH
14 PCH
PC
6⁷
PCLATH
PCL
0
GOTO, CALL
0 11
OPCODE <10:0>
PCL 0
CALLW
0
8
W
14 PCH
PC
PCL 0
BRW
15
PC + W
14 PCH
PC
PCL 0
BRA
15
PC + OPCODE <8:0>
2.3.1 MODIFYING PCL
Executing any instruction with the PCL register as the
destination simultaneously causes the Program Coun-
ter PC<14:8> 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 7 bits to the PCLATH register. When the
lower 8 bits are written to the PCL register, all 15 bits of
the program counter will change to the values con-
tained in the PCLATH register and those being written
to the PCL register.
2.3.2 COMPUTED GOTO
A computed GOTO is accomplished by adding an offset to
the program counter (ADDWF PCL). When performing 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).
2.3.3 COMPUTED FUNCTION CALLS
A computed function CALL allows programs to maintain
tables of functions and provide another way to execute
state machines or look-up tables. When performing a
table read using a computed function CALL, care
should be exercised if the table location crosses a PCL
memory boundary (each 256-byte block).
If using the CALL instruction, the PCH<2:0> and PCL
registers are loaded with the operand of the CALL
instruction. PCH<6:3> is loaded with PCLATH<6:3>.
The CALLW instruction enables computed calls by com-
bining PCLATH and W to form the destination address.
A computed CALLW is accomplished by loading the W
register with the desired address and executing CALLW.
The PCL register is loaded with the value of W and
PCH is loaded with PCLATH.
2.3.4 BRANCHING
The branching instructions add an offset to the PC.
This allows relocatable code and code that crosses
page boundaries. There are two forms of branching,
BRW and BRA. The PC will have incremented to fetch
the next instruction in both cases. When using either
branching instruction, a PCL memory boundary may be
crossed.
If using BRW, load the W register with the desired
unsigned address and execute BRW. The entire PC will
be loaded with the address PC + 1 + W.
If using BRA, the entire PC will be loaded with PC + 1 +,
the signed value of the operand of the BRA instruction.
2.4 Stack
All devices have a 16-level x 15-bit wide hardware
stack (refer to Figures 2-1 and 2-3). The stack space is
not part of either program or data space. The PC is
PUSHed onto the stack when CALL or CALLW instruc-
tions are 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, if the STVREN
bit = 0 (Configuration Word 2). This means that after
the stack has been PUSHed sixteen times, the seven-
teenth PUSH overwrites the value that was stored from
the first PUSH. The eighteenth PUSH overwrites the
second PUSH (and so on).
Note 1: There are no instructions/mnemonics
called PUSH or POP. These are actions
that occur from the execution of the
CALL, CALLW, RETURN, RETLW and
RETFIE instructions or the vectoring to
an interrupt address.
DS41364A-page 52
Preliminary
© 2008 Microchip Technology Inc.