PIC16C432-I データシートの表示（PDF） - Microchip Technology

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PIC16C432-I

OTP 8-Bit CMOS MCU with LIN Transceiver

Microchip Technology 

PIC16C432

4.3 I/O Programming Considerations

4.3.1 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 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

(i.e., 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

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.

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

(i.e., 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 4-2 shows the effect of two sequential read-

modify-write instructions (i.e., BCF, BSF, etc.) on an

I/O port.

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.

EXAMPLE 4-2:

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-up 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 pppp. The 2nd BCF caused RB7 to be latched as

; the pin value (High).

4.3.2

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 4-

7). 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 volt-

age to stabilize (load dependent) before the next

instruction causes that file to be read into the CPU.

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 4-7:

SUCCESSIVE I/O OPERATION

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

PCPC

InIsntrsutcrutiocntion

feFtechtcehded

PC

MMOOVVWWFF PPOORRTTBB

Write to

PORTB

PC + 1

MMOOVVFF PPOORRTTBB,,WW

Read PORTB

PC + 2

NONPOP

PC + 3

NNOOPP

RRBB<<77:0:0>>

T PD

EExxeeccuuttee

MMOOVVWWFF

PPOORRTTBB

Port pin

sampled here

EExxeeccute

MMOOVVFF

PPOORRTTBB,, WW

EExxeeccuuttee

NNOOP

Note:

This example shows write to PORTB

followed by a read from PORTB.

Note that: 

data setup time = (0.25 TCY - TPD)

where TCY = instruction cycle and

TPD = propagation delay of Q1 cycle

to output valid.

Therefore, at higher clock frequencies,

a write followed by a read may be

problematic.

DS41140C-page 22

Preliminary

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