DB929B datasheet(19/51 Pages) CMLMICRO

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4-Level FSK Modem Data Pump

Page 19 of 50

MX929B PRELIMINARY INFORMATION

www.mxcom.com Tel: 800 638 5577 336 744 5050

Fax: 336 744 5054

Doc. # 20480171.003

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA

All trademarks and service marks are held by their respective companies.

RXIN Signal

for Task 1

for Task 2

IRQ Output (IRQEN = '1')

IRQ Bit of Status Register

BFREE Bit of Status Register

Task 1

Task 2

Task 1 data

Data from Block Buffer to C

Task from C to Command

Figure 10: Receive Task Overlapping

Detailed timings for the various tasks are provide in Figure 11 and Figure 12.

MX929B Modem Tasks:

Receive Mode

Transmit Mode

NULL

SFP

Search for Frame Preamble

T24S

Transmit 24 symbols

RHB

Read Header Block

THB

Transmit Header Block

RILB

Read Intermediate or Last Block

TIB

Transmit Intermediate Block

SFS

Search for Frame Sync

TLB

Transmit Last Block

R4S

Read 4 symbols

T4S

Transmit 4 symbols

RSID

Read Station ID

TSID

Transmit Station ID

RESET

Cancel any current action

RESET

Cancel any current action

4.5.2.7

NULL: No effect

This is provided so an AQSC or AQLEV command can be initiated without loading a new task.

4.5.2.8

SFP: Search for Frame Preamble

This task causes the modem to search the received signal for a valid 24-symbol Frame Preamble, consisting

of a 24-symbol Frame Sync sequence followed by Station ID Block which has a correct CRC0 checksum.

The task continues until a valid Frame Preamble has been found.

The search consists of four stages:

First the modem will attempt to match the incoming symbols against the Frame Synchronization pattern

to within the tolerance defined by the FSTOL bits of the Control Register.

Once a match has been found, the modem will read in the following 'S' symbol, place it in the SVAL bits

of the Status Register then set the SRDY bit to '1'. (The IRQ bit of the Status Register will also be set to

'1' at this time if the SSIEN bit of the Mode Register is '1').

The modem will then read the next 22 symbols as station ID data. They will be decoded and the CRC0

checked. If this is incorrect, the modem will resume the search, looking for a fresh Frame Sync pattern.

If the received CRC0 is correct, the following 'S' symbol will be read into the SVAL bits of the Status

decoded Station ID bytes placed into the Data Block Buffer.

Upon detecting that the BFREE bit of the Status Register has gone to '1', the µC should read the three Station

ID bytes from the Data Block Buffer and then write the next task to the modem's Command Register.

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