參數(shù)資料
型號: PIC16C662T-20I/L
元件分類: 微控制器/微處理器
英文描述: 8-BIT, OTPROM, 20 MHz, RISC MICROCONTROLLER, PQCC44
封裝: PLASTIC, LCC-44
文件頁數(shù): 57/136頁
文件大?。?/td> 634K
代理商: PIC16C662T-20I/L
1996 Microchip Technology Inc.
Preliminary
DS30559A-page 27
PIC16C64X & PIC16C66X
4.3
PCL and PCLATH
The program counter (PC) is 13-bits wide. The low byte
comes from the PCL register, which is readable and
writable. The high byte (PC<12:8>) is not directly read-
able or writable and comes from PCLATH. On any
reset, the PC is cleared. Figure 4-11 shows the two
situations for the loading of the PC. The upper example
in the gure shows how the PC is loaded on a write to
PCL (PCLATH<4:0>
→ PCH). The lower example in
the gure shows how the PC is loaded during a CALL
or GOTO instruction (PCLATH<4:3>
→ PCH).
FIGURE 4-11: LOADING OF PC IN
DIFFERENT SITUATIONS
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
“Implementing a Table Read”
(AN556).
PC
12
8
7
0
5
PCLATH<4:0>
PCLATH
Instruction with
ALU result
GOTO, CALL
Opcode <10:0>
8
PC
12
11 10
0
11
PCLATH<4:3>
PCH
PCL
87
2
PCLATH
PCH
PCL
PCL as
Destination
4.3.2
STACK
PIC16C64X & PIC16C66X devices have an 8 level
deep x 13-bit wide hardware stack (Figure 4-2). 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 rst
push. The tenth push overwrites the second push (and
so on).
4.4
Program Memory Paging
PIC16C642 and PIC16C662 devices have 4K of pro-
gram memory, but the CALL and GOTO instructions only
have an 11-bit address range. This 11-bit address
range allows a branch within a 2K program memory
page size. To allow CALL and GOTO instructions to
address the entire 4K program memory address range,
there must be another bit to specify the program mem-
ory page. This paging bit comes from the PCLATH<3>
bit (Figure 4-11). When doing a CALL or GOTO instruc-
tion, the user must ensure that this page select bit
(PCLATH<3>) is programmed so that the desired pro-
gram memory page is addressed. If a return from a
CALL
instruction (or interrupt) is executed, the entire
13-bit PC is pushed onto the stack. Therefore, manipu-
lation of the PCLATH<3> bit is not required for the
return instructions (which POPs the address from the
stack).
Note 1: There are no status bits to indicate stack
overow or stack underow conditions.
Note 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 instruc-
tions, or the vectoring to an interrupt
address.
Note:
The PIC16C64X & PIC16C66X ignore the
PCLATH<4> bit, which is used for program
memory pages 2 and 3 (1000h - 1FFFh).
The use of PCLATH<4> as a general pur-
pose read/write bit is not recommended
since this may affect upward compatibility
with future products.
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