PIC16LC505-20SLASHP datasheet(20/80 Pages) MICROCHIP

20 / 80 page

PIC16C505

DS40192A-page 20

Preliminary

TABLE 5-1:

SUMMARY OF PORT REGISTERS

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1 Bit 0

Value on

Power-On

Reset

Value on

MCLR and

WDT Reset

Value on

Wake-up on

Pin Change

N/A

TRISB

—

I/O control registers

--11 1111

N/A

TRISC

—

I/O control registers

--11 1111

N/A

OPTION

RBWU

RBPU

TOCS

TOSE

PSA

PS2

PS1

PS0

1111 1111

03h

STATUS

RBWUF

—

PAO

0001 1xxx

000q quuu

100q quuu

06h

PORTB

—

RB5

RB4

RB3

RB2

RB1

RB0

--xx xxxx

--uu uuuu

07h

PORTC

—

RC5

RC4

RC3

RC2

RC1

RC0

--xx xxxx

--uu uuuu

Legend: Shaded cellls not used by Port Registers, read as ‘0’, — = unimplemented, read as ‘0’, x = unknown, u = unchanged.

5.5

I/O Programming Considerations

5.5.1

BI-DIRECTIONAL I/O PORTS

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.

EXAMPLE 5-1:

READ-MODIFY-WRITE

INSTRUCTIONS ON AN

I/O PORT

;Initial PORTB Settings

; PORTB<5:3> Inputs

; PORTB<2:0> Outputs

;

PORTB latch

PORTB pins

;

----------

BCF

PORTB, 5

;--01 -ppp

--11 pppp

BCF

PORTB, 4

;--10 -ppp

--11 pppp

MOVLW 007h

;

TRIS

PORTB

;--10 -ppp

--11 pppp

;

;Note that the user may have expected the pin

;values to be --00 pppp. The 2nd BCF caused

;RB5 to be latched as the pin value (High).

5.5.2

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 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.

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