HT9480 datasheet(24/57 Pages) HOLTEK

24 / 57 page

A codeword is either an address or a message

codeword. Idle codewords are transmitted to

fill in empty batches or to separate messages.

An address codeword is coded as shown above.

Of the 21 bits of user addresses, 18 bits are

coded in the codeword itself (bits 2 to 19),

which is protected against transmission er-

rors by a number of CRC checkbits (bits 22 to

31). Bit 32 is an overall even-parity bit.

The two function bits (bits 20 and 21) allow

distinction of four different calls to one user ad-

dress as shown in following Table.

Bit 20

(MSB)

Bit 21

(LSB)

Call Type

Data Format

Numeric

4-bits per digit

Alert only

—

Alert only

—

Alpha-numeric

7-bits per ASCII

character

An idle codeword is a valid address codeword,

which cannot be allocated to the pager.

There is a total of 20 bits of caller information

to be put into a message codeword (bits 2 to 21),

which is protected by the CRC checkbits (bits 22 to

31).

• Decoding of the POCSAG data stream

The POCSAG coded input data received from

RF module is first filtered by an internal digi-

tal filter in the decoder. From the filtered

data, a sampling clock synchronous to the

data rate is derived. The decoder supports

512, 1200, and 2400 bits per second data rate,

which in turn results in their corresponding

sampling clock frequency.

Upon detection of a valid call, the decoder

performs several operations (refer to the fol-

lowing section of the Message Data Transfer).

Call termination is normally deemed when a

valid idle or another address codeword is re-

ceived after a message code word.

• Erroneous codewords

Upon receipt of erroneous uncorrectable code-

words, call termination occurs according to

the conditions given below:

SPF08 SPF09 Call Termination Event

Any two consecutive

codewords or the

codeword directly

following the address

codeword in error

Any single codeword in error

Any two consecutive

codewords in error

Error correction

Item

Description

Preamble

4 random errors in 31 bits

Synchronization

code-word

2 random errors in 32 bits

Address

code-word

2 random errors, or 4-bit

burst errors (optional)

Message

code-word

2 random errors, or 4-bit

burst errors (optional)

In the HT9480 error correction methods have

been implemented as shown in above Table.

Random error correction is default for both ad-

dress and message code-words. Burst error cor-

rection can be switched by SPF 15. Up to 4 bits

of burst errors can be corrected.

Decoder interface

The HT9480 has two interfaces available. One

is the pager control address (1EH), which con-

trols the operation and configuration of the de-

coder. The other is the pager data address

(1FH), which places the message data of calls in

the parallel mode.

HT9480

23th Feb ’98

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