HT656 datasheet(5/14 Pages) HOLTEK

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Functional Description

Operation

The 3

18 series of decoders provides various com-

binations of address and data pins in different

packages. It is paired with the 3

18 series of

encoders. The decoders receive data transmit-

ted by the encoders and interpret the first N

bits of the code period as address and the last

18–N bits as data (where N is the address code

number). A signal on the DIN pin then activates

the oscillator which in turns decodes the incom-

ing address and data. The decoders will check

the received address twice continuously. If all

the received address codes match the contents

of the decoder’s local address, the 18–N bits of

data are decoded to activate the output pins,

and the VT pin is set high to indicate a valid

transmission. That will last until the address

code is incorrect or no signal has been received.

The output of the VT pin is high only when the

transmission is valid. Otherwise it is low al-

ways.

Output type

There are 2 types of output to select from:

• Momentary type

The data outputs follow the encoder during a

valid transmission and then reset.

• Latch type

The data outputs follow the encoder during a

valid transmission, and are then latched in

this state until the next valid transmission

occurs.

Flowchart

Note: The oscillator is disabled in the

standby state and activated as long as

a logic “high” signal is applied to the

DIN pin. i.e., the DIN should be kept

“low” if there is no signal input.

18 Series of Decoders

2nd Oct ’97

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