PCD5002USLASH10 datasheet(10/48 Pages) PHILIPS

10 / 48 page

1997 Jun 24

Philips Semiconductors

Product speciﬁcation

Advanced POCSAG and APOC-1 Paging

Decoder

PCD5002

When a message extends beyond the end of a batch no

testing for sync takes place. Instead, a message data

transfer will be initiated by an interrupt to the external

controller. Data reception continues normally after a period

corresponding to the sync word duration.

If any message codeword is found to be uncorrectable, the

‘data fail’ mode is entered and no data transfer will be

attempted at the next sync word position. Instead, a test for

sync word will be carried out.

In the data fail mode message reception continues

normally for 1 batch duration. When a sync word is

detected at the expected position the decoder returns to

the ‘data receive’ mode. If the sync word again fails to

appear, then batch synchronization is deemed lost. Call

reception is then terminated and the ‘fade recovery’ mode

is entered.

The fade recovery mode is intended to scan for sync word

and preamble over an extended window (nominal

position

± 8 bits). This is performed for a period of up to

15 batches, allowing recovery of synchronization from

long fades in the radio signal. Detection of preamble

causes switching to the ‘preamble receive’ mode, while

sync word detection causes switching to the ‘data receive’

mode. When neither is found within a period of 15 batches,

the radio signal is considered lost and the ‘carrier off’ mode

is entered.

The purpose of the carrier off mode is to detect a valid

radio transmission and synchronize to it quickly and

efficiently. Because transmissions may start at random,

the decoder enables the receiver for 1 codeword in every

18 codewords looking for preamble or sync word.

By using a buffer containing 32 bits (n bits from the current

scan, 32

−n from the previous scan) effectively every batch

bit position can be tested within a continuous transmission

of at least 18 batches. Detection of preamble causes the

device to switch to the ‘preamble receive’ mode, while

sync word detection causes the device to switch to the

‘data receive’ mode.

Fig.4 ACCESS® synchronization algorithm for POCSAG.

handbook, full pagewidth

OFF to ON status

preamble

sync word

no sync word

sync word

preamble

sync word

preamble

sync word

no preamble or

sync word

(3 batches)

no preamble or

sync word

no preamble or

sync word

(1 batch)

no preamble or

sync word

(15 batches)

switch-on

preamble receive

data receive

data fail

fade recovery

carrier off

(1 batch)

MLC247

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