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參數(shù)資料
| 型號： | PIC16C74A-04I/P |
| 廠商： | Microchip Technology |
| 文件頁數(shù)： | 64/114頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC MCU OTP 4KX14 A/D PWM 40DIP |
| 產(chǎn)品培訓模塊： | Asynchronous Stimulus 8-bit PIC® Microcontroller Portfolio |
| 標準包裝： | 10 |
| 系列： | PIC® 16C |
| 核心處理器： | PIC |
| 芯體尺寸： | 8-位 |
| 速度： | 4MHz |
| 連通性： | I²C，SPI，UART/USART |
| 外圍設備： | 欠壓檢測/復位，POR，PWM，WDT |
| 輸入/輸出數(shù)： | 33 |
| 程序存儲器容量： | 7KB（4K x 14） |
| 程序存儲器類型： | OTP |
| RAM 容量： | 192 x 8 |
| 電壓 - 電源 (Vcc/Vdd)： | 4 V ~ 6 V |
| 數(shù)據(jù)轉換器： | A/D 8x8b |
| 振蕩器型： | 外部 |
| 工作溫度： | -40°C ~ 85°C |
| 封裝/外殼： | 40-DIP（0.600"，15.24mm） |
| 包裝： | 管件 |
| 配用： | 444-1001-ND - DEMO BOARD FOR PICMICRO MCU |
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1997 Microchip Technology Inc.
DS30390E-page 53
PIC16C7X
5.6
I/O Programming Considerations
5.6.1
BI-DIRECTIONAL I/O PORTS
Any instruction which writes, operates internally as a
read followed by a write operation. The BCF and BSF
instructions, for example, read the register into the
CPU, execute the bit operation and write the result back
to the register. Caution must be used when these
instructions are applied to a port with both inputs and
outputs dened. For example, a BSF operation on bit5
of PORTB will cause all eight bits of PORTB to be read
into the CPU. Then the BSF operation takes place on
bit5 and PORTB is written to the output latches. If
another bit of PORTB is used as a bi-directional I/O pin
(e.g., bit0) and it is dened as an input at this time, the
input signal present on the pin itself would be read into
the CPU and rewritten to the data latch of this particular
pin, overwriting the previous content. As long as the pin
stays in the input mode, no problem occurs. However, if
bit0 is switched to an output, the content of the data
latch may now be unknown.
Reading the port register, reads the values of the port
pins. Writing to the port register writes the value to the
port latch. When using read-modify-write instructions
(ex. BCF, BSF, etc.) on a port, the value of the port pins
is read, the desired operation is done to this value, and
this value is then written to the port latch.
Example 5-4 shows the effect of two sequential read-
modify-write instructions on an I/O port.
Applicable Devices
72 73 73A 74 74A 76 77
EXAMPLE 5-4:
READ-MODIFY-WRITE
INSTRUCTIONS ON AN I/O
PORT
;Initial PORT settings: PORTB<7:4> Inputs
;
PORTB<3:0> Outputs
;PORTB<7:6> have external pull-ups and are
;not connected to other circuitry
;
PORT latch
PORT pins
;
----------
---------
BCF PORTB, 7
; 01pp pppp
11pp pppp
BCF PORTB, 6
; 10pp pppp
11pp pppp
BSF STATUS, RP0
;
BCF TRISB, 7
; 10pp pppp
11pp pppp
BCF TRISB, 6
; 10pp pppp
10pp pppp
;
;Note that the user may have expected the
;pin values to be 00pp ppp. The 2nd BCF
;caused RB7 to be latched as the pin value
;(high).
A pin actively outputting a Low or High should not be
driven from external devices at the same time in order
to change the level on this pin (“wired-or”, “wired-and”).
The resulting high output currents may damage the
chip.
5.6.2
SUCCESSIVE OPERATIONS ON I/O PORTS
The actual write to an I/O port happens at the end of an
instruction cycle, whereas for reading, the data must be
valid at the beginning of the instruction cycle (Figure 5-
10). Therefore, care must be exercised if a write fol-
lowed by a read operation is carried out on the same I/
O port. The sequence of instructions should be such to
allow the pin voltage to stabilize (load dependent)
before the next instruction which causes that le to be
read into the CPU is executed. Otherwise, the previous
state of that pin may be read into the CPU rather than
the new state. When in doubt, it is better to separate
these instructions with a NOP or another instruction not
accessing this I/O port.
FIGURE 5-10: SUCCESSIVE I/O OPERATION
PC
PC + 1
PC + 2
PC + 3
Q1 Q2 Q3 Q4
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
Q1 Q2 Q3 Q4
Instruction
fetched
RB7:RB0
MOVWF PORTB
write to
PORTB
NOP
Port pin
sampled here
NOP
MOVF PORTB,W
Instruction
executed
MOVWF PORTB
write to
PORTB
NOP
MOVF PORTB,W
PC
TPD
Note:
This example shows a write to PORTB
followed by a read from PORTB.
Note that:
data setup time = (0.25TCY - TPD)
where TCY = instruction cycle
TPD = propagation delay
Therefore, at higher clock frequencies,
a write followed by a read may be prob-
lematic.
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