SX28ACSLASHDP datasheet(20/51 Pages) PARALLAX

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Parallax SX20AC/SX28AC

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Page 20 of 51

Rev 1.6 11/20/2006

7.2.

Port B MIWU/Interrupt Configuration

The WKPND_B register comes up with an unknown

value upon reset. The user program must clear the register

prior to enabling the wake-up condition or interrupts. The

proper initialization sequence is:

1. Select the desired edge (through WKED_B

2. Clear the WKPND_B register.

3. Enable

the

Wakeup

condition

(through

WKEN_B register).

Below is an example of how to read the WKPND_B

or interrupt, and to clear the WKPND_B register:

mov

M,#$09

clr

mov

RB,W

;W contains WKPND_B

;contents of W exchanged

;with contents of WKPND_B

The final “mov” instruction in this example performs an

exchange of data between the working register (W) and

the WKPND_B register. This exchange occurs only with

Port B accesses. Otherwise, the “mov” instruction does

not perform an exchange, but only moves data from the

source to the destination.

Here is an example of a program segment that configures

the RB0, RB1, and RB2 pins to operate as Multi Input

Wakeup/Interrupt pins, sensitive to falling edges:

mov

M,#$0F

;prepare to write port data

;direction registers

mov

W,#$07

;load W with the value 07h

mov

RB,W

;configure RB0-RB2 to be inputs

mov

M,#$0A

;prepare to write WKED_B

;(edge) register

;W contains the value 07h

mov

!RB,W

;configure RB0-RB2 to sense

;falling edges

mov

M,#$09

;prepare to access WKPND_B

;(pending) register

mov

W,#$00

;clear W

mov

!RB,W

;clear all wakeup pending bits

mov

M,#$0B

;prepare to write WKEN_B (enable)

;register

mov

W,#$F8h ;load W with the value F8h

mov

!RB,W

;enable RB0-RB2 to operate as

;wakeup inputs

To prevent false interrupts, the enabling step (clearing bits

in WKEN_B) should be done as the last step in a

sequence of Port B configuration steps. After this

program segment is executed, the device can receive

interrupts on the RB0, RB1, and RB2 pins. If the device is

put into the power down mode (by executing the SLEEP

instruction), the device can then receive wakeup signals

on those same pins.

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