Microchip Technology IC MCU OTP 2KX PIC17C42A-16/P DIP-40 MCP PIC17C42A-16/P Data Sheet

DS30412C-page 64



 1996 Microchip Technology Inc.

9.5.1

BI-DIRECTIONAL I/O PORTS

Any instruction which writes, operates internally as a
read followed by a write operation. For example, the

BCF

 and 

BSF

 instructions 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 defined. 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 defined as an input at this time, the
input signal present on the pin itself would be read into
the CPU and re-written to the data latch of this particu-
lar pin, overwriting the previous content. As long as the
pin stays in the input mode, no problem occurs. How-
ever, if bit0 is switched into output mode later on, the
content of the data latch may now be unknown.

Reading a port 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 (

BCF, BSF

,

BTG

, etc.) on a port, the value of the port pins is read,

the desired operation is performed with this value, and
the value is then written to the port latch. 

9-5 shows the effect of two sequential

read-modify-write instructions on an I/O port.

  

; Initial PORT settings: PORTB<7:4> Inputs

;                        PORTB<3:0> Outputs

; PORTB<7:6> have 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

;

   BCF   DDRB, 7        10pp pppp   11pp pppp

   BCF   DDRB, 6        10pp pppp   10pp pppp

;

; 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). 

   

9.5.2

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 9-
9). 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 be such to allow
the pin voltage to stabilize (load dependent) before
executing the instruction that reads the values on that
I/O port. 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.

A pin actively outputting a Low or High
should not be driven from external devices
in order to change the level on this pin (i.e.
“wired-or”, “wired-and”). The resulting high
output currents may damage the device.

Note:

This example shows a write to PORTB
followed by a read from PORTB.
Note that:
data setup time = (0.25 T

CY

 - T

PD

)

where T

CY

 = instruction cycle.

Therefore, at higher clock 
frequencies, a write followed by a 
read may be problematic.

           T

PD

 = propagation delay

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