PIC16F688-ISLASHML datasheet(36/204 Pages) MICROCHIP

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PIC16F688

DS41203E-page 34

4.2

Additional Pin Functions

Every PORTA pin on the PIC16F688 has an interrupt-

on-change option and a weak pull-up option. PORTA

also provides an Ultra Low-Power Wake-up option. The

next three sections describe these functions.

4.2.1

ANSEL REGISTER

The ANSEL register is used to configure the Input

mode of an I/O pin to analog. Refer to Register 4-3.

Setting the appropriate ANSEL bit high will cause all

digital reads on the pin to be read as ‘0’ and allow

analog functions on the pin to operate correctly.

The state of the ANSEL bits has no affect on digital

output functions. A pin with TRIS clear and ANSEL set

will still operate as a digital output, but the Input mode

will be analog. This can cause unexpected behavior

when executing read-modify-write instructions on the

affected port.

4.2.2

WEAK PULL-UPS

Each of the PORTA pins, except RA3, has an

individually configurable internal weak pull-up. Control

bits WPUAx enable or disable each pull-up. Refer to

off when the port pin is configured as an output. The

pull-ups are disabled on a Power-on Reset by the

RAPU bit of the OPTION register. A weak pull-up is

automatically enabled for RA3 when configured as

MCLR and disabled when RA3 is an I/O. There is no

software control of the MCLR pull-up.

4.2.3

INTERRUPT-ON-CHANGE

Each of the PORTA pins is individually configurable as

an interrupt-on-change pin. Control bits IOCAx enable

or disable the interrupt function for each pin. Refer to

Power-on Reset.

For enabled interrupt-on-change pins, the values are

compared with the old value latched on the last read of

PORTA. The ‘mismatch’ outputs of the last read are

OR’d together to set the PORTA Change Interrupt Flag

bit (RAIF) in the INTCON register.

This interrupt can wake the device from Sleep. The user,

in the Interrupt Service Routine, clears the interrupt by:

Any read or write of PORTA. This will end the

mismatch condition, then

Clear the flag bit RAIF.

A mismatch condition will continue to set flag bit RAIF.

Reading PORTA will end the mismatch condition and

allow flag bit RAIF to be cleared. The latch holding the

last read value is not affected by a MCLR nor BOR

Reset. After these Resets, the RAIF flag will continue

to be set if a mismatch is present.

Note:

If a change on the I/O pin should occur

when the read operation is being executed

(start of the Q2 cycle), then the RAIF

interrupt flag may not get set.

ANSEL: ANALOG SELECT REGISTER

R/W-1

ANS7

ANS6

ANS5

ANS4

ANS3

ANS2

ANS1

ANS0

bit 7

bit 0

Legend:

R = Readable bit

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

bit 7-0

ANS<7:0>: Analog Select bits

Analog select between analog or digital function on pins AN<7:0>, respectively.

= Analog input. Pin is assigned as analog input(1).

= Digital I/O. Pin is assigned to port or special function.

Note 1:

Setting a pin to an analog input automatically disables the digital input circuitry, weak pull-ups and

interrupt-on-change, if available. The corresponding TRIS bit must be set to Input mode in order to allow

external control of the voltage on the pin.

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