PIC16C54B-04I/SO Microchip Technology, PIC16C54B-04I/SO Datasheet - Page 26

• Manufacturer Part Number: PIC16C54B-04I/SO
• Description: MICRO CTRL 512 4MHZ OTP 18SOIC
• Manufacturer: Microchip Technology
• Series: PIC® 16Cr

Specifications of PIC16C54B-04I/SO

• Core Processor: PIC
• Core Size: 8-Bit
• Speed: 4MHz
• Peripherals: POR, WDT
• Number Of I /o: 12
• Program Memory Size: 768B (512 x 12)
• Program Memory Type: OTP
• Ram Size: 25 x 8
• Voltage - Supply (vcc/vdd): 3 V ~ 5.5 V
• Oscillator Type: External
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 18-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Request inventory verification / Request inventory verification
• Eeprom Size: -
• Data Converters: -
• Connectivity: -

PIC16C5X
5.6
5.6.1
Some instructions operate internally as read followed
by write operations. The BCF and BSF instructions, for
example, read the entire port into the CPU, execute
the bit operation and re-write the result. Caution must
be used when these instructions are applied to a port
where one or more pins are used as input/outputs. For
example, a BSF operation on bit5 of PORTB will cause
all eight bits of PORTB to be read into the CPU, bit5 to
be set and the PORTB value to be written to the output
latches. If another bit of PORTB is used as a
bi-directional I/O pin (say bit0) and it is defined 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
into output mode later on, the content of the data latch
may now be unknown.
Example 5-1 shows the effect of two sequential
read-modify-write instructions (e.g., BCF, BSF, etc.) on
an I/O port.
A pin actively outputting a high or a low 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.
FIGURE 5-2:
DS30453B-page 26
Instruction
Instruction
I/O Programming Considerations
BI-DIRECTIONAL I/O PORTS
executed
RB7:RB0
fetched
SUCCESSIVE I/O OPERATION
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
MOVWF PORTB
PC
MOVF PORTB,W
MOVWF PORTB
(Write to
PORTB)
Port pin
written here
PC + 1
MOVF PORTB,W
Preliminary
(Read
Port pin
sampled here
PORTB)
NOP
PC + 2
EXAMPLE 5-1:
;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
;
;
;
;
;Note that the user may have expected the pin
;values to be 00pp pppp. The 2nd BCF caused
;RB7 to be latched as the pin value (High).
5.6.2
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 5-2). Therefore, care must be exercised if a
write followed by a read operation is carried out on the
same I/O port. The sequence of instructions should
allow the pin voltage to stabilize (load dependent)
before the next instruction, which causes that file 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.
BCF
BCF
MOVLW 03Fh
TRIS
PC + 3
NOP
NOP
SUCCESSIVE OPERATIONS ON I/O
PORTS
PORTB, 7
PORTB, 6
PORTB
This example shows a write
to PORTB followed by a read
from PORTB.
READ-MODIFY-WRITE
INSTRUCTIONS ON AN
I/O PORT
;01pp pppp
;10pp pppp
;
;10pp pppp
PORT latch
----------
1998 Microchip Technology Inc.
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PORT pins
11pp pppp
11pp pppp
10pp pppp