Data Sheet

4-Level FSK Modem Data Pump Page 12 of 47 MX919B PRELIMINARY INFORMATION

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4.1.13 Rx Level/Clock Extraction

These circuits, which operate only in receive mode, derive a symbol rate clock from the received signal and

measure the received signal amplitude and DC offset. This information is then used to extract the received 4-

level symbols and also to provide an input to the received Data Quality measuring circuit. The external

capacitors C6 and C7 form part of the received signal level measuring circuit.

The capacitors C6 and C7 are driven from a very high impedance source so any measurement of the

voltages on the DOC pins must be made via high input impedance (MOS input) voltage followers to avoid

disturbance of the level measurement circuits.

Further details of the level and clock extraction functions are given in Section 5.3.

4.1.14 Clock Oscillator and Dividers

These circuits derive the transmit symbol rate (and the nominal receive symbol rate) by frequency division of

a reference frequency which may be generated by the on-chip Xtal oscillator or applied from an external

source.

Note: If the on-chip Xtal oscillator is to be used, then the external components X1, C3, C4, and R3 are

required. If an external clock source is to be used, then it should be connected to the XTAL/CLOCK

input pin, the

XTAL pin should be left unconnected, and X1, C3, C4, and R3 should not be installed.

4.2 Modem - µC Interaction

In general, data is transmitted over-air in the form of messages, or 'Frames', consisting of a 'Frame Preamble'

followed by one or more formatted data blocks. The Frame Preamble includes a Frame Synchronization

pattern designed to allow the receiving modem to identify the start of a frame. The following data blocks are

constructed from the 'raw' data using a combination of CRC (Cyclic Redundancy Checksum) generation,

Forward Error Correction coding, and Interleaving. Details of the message formats handled by the modem

are provided in Section 4.4, Figure 7, and Figure 8.

To reduce the processing load on the associated µC, the MX919B modem has been designed to perform as

much of the computationally intensive work involved in Frame formatting and de-formatting and (when in

receive mode) searching for and synchronizing onto the Frame Preamble. In normal operation, the modem

will only require servicing by the µC once per received or transmitted block.

Therefore, to transmit a block, the controlling µC needs only to load the unformatted 'raw' binary data into the

modem's Data Block Buffer, then instruct the modem to format and transmit that data. The modem will then

calculate and add the CRC bits as required, encode the result as 4-level symbols (with Forward Error

Correction coding), and interleave the symbols before transmission.

In receive mode, the modem can be instructed to assemble a block's worth of received symbols, de-interleave

the symbols, translate them to binary, perform Forward Error Correction, and check the resulting CRC before

placing the received binary data into the Data Block Buffer for the µC to read.

The modem can also handle the transmission and reception of unformatted data using the T4S, T24S, and

R4S tasks as described in Sections 4.3 and 4.5.2. These tasks are normally used for the transmission of

Symbol and Frame Synchronization sequences. These tasks may also be used for the transmission and

reception of special test patterns or special data formats. In such a case, care should be taken to ensure that

the transmitted TXOUT signal contains enough level and timing information for the receiving modem's level

and clock extraction circuits to function correctly.

See Section 5.3.