參數資料
型號： PIC16CR7X
廠商： Microchip Technology Inc.
英文描述： 28/40-Pin, 8-Bit CMOS ROM Microcontrollers
中文描述： 28/40-Pin，8位微控制器的CMOS光盤
文件頁數： 28/172頁
文件大?。?/td> 1506K
代理商： PIC16CR7X
PIC16CR7X
DS21993C-page 26
2007 Microchip Technology Inc.
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 2-1 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
GOTO
instruction (PCLATH<4:3>
→
PCH).
FIGURE 2-4:
LOADING OF PC IN
DIFFERENT SITUATIONS
2.3.1
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,
“Implementing a Table Read”
(AN556).
2.3.2
STACK
The PIC16CR7X 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 inter-
rupt causes a branch. The stack is POPed in the event
of a
RETURN, RETLW
or a
RETFIE
instruction execu-
tion. 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).
2.4
Program Memory Paging
PIC16CR7X devices are capable of addressing a con-
tinuous 8K word block of program memory. The
CALL
and
GOTO
instructions provide only 11 bits of address to
allow branching within any 2K program memory page.
When doing a
CALL
or
GOTO
instruction, the upper 2
bits of the address are provided by PCLATH<4:3>.
When doing a
CALL
or
GOTO
instruction, the user must
ensure that the page select bits are programmed so
that the desired program memory page is addressed. If
a return from a
CALL
instruction (or interrupt) is exe-
cuted, the entire 13-bit PC is popped off the stack.
Therefore, manipulation of the PCLATH<4:3> bits are
not required for the
RETURN
instructions (which POPs
the address from the stack).
Example 2-1 shows the calling of a subroutine in
page 1 of the program memory. This example assumes
that PCLATH is saved and restored by the Interrupt
Service Routine
(if interrupts are used).
EXAMPLE 2-1:
CALL OF A SUBROUTINE
IN PAGE 1 FROM PAGE 0
0x500
PCLATH,4
PCLATH,3 ;Select page 1
;(800h-FFFh)
CALL SUB1_P1
;Call subroutine in
:
;page 1 (800h-FFFh)
:
ORG
0x900
;page 1 (800h-FFFh)
PC
12
8
7
0
5
PCLATH<4:0>
PCLATH
Instruction with
PCL as
Destination
ALU
GOTO,CALL
Opcode <10:0>
8
PC
12
11 10
0
11
PCLATH<4:3>
PCH
PCL
8
7
2
PCLATH
PCH
PCL
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.
Note:
The contents of the PCLATH are
unchanged after a
RETURN
or
RETFIE
instruction is executed. The user must
setup the PCLATH for any subsequent
CALLS
or
GOTOS
.
ORG
BCF
BSF
SUB1_P1
:
:
:
;called subroutine
;page 1 (800h-FFFh)
RETURN
;return to Call
;subroutine in page 0
;(000h-7FFh)
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