GMSK Modem Data Pump
Page 9 of 37
MX909A PRELIMINARY INFORMATION
4.1.14 Clock Oscillator and Dividers
This circuit derives the transmit bit rate (and the nominal receive bit 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 external components X1, C3, C4 and R3 are required.
If an external clock source is to be used, then the external clock source should be connected to the
XTAL/CLOCK input pin, the XTAL pin should be left unconnected, and X1, C3, C4, and R3 not fitted.
4.1.15 Scramble/De-scramble
This block may be used to scramble/de-scramble the transmitted/received data blocks. It does this by
modulating the data with a 511-bit pseudorandom sequence, as described in section 4.5.4, smoothes the
transmitted spectrum, especially when repetitive sequences are to be transmitted.
4.2 Modem - µC Interaction
In general, data is transmitted over air in the form of messages, or 'Frames', consisting of a 'Frame Head'
optionally followed by one or more formatted data blocks. The Frame Head 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, Interleaving and Scrambling. Details of the message formats
handled by this modem are given in section 4.3.
To reduce the processing load on the host µC, this modem has been designed to perform as much as
possible of the computationally intensive work involved in Frame formatting / de-formatting and (when in
receive mode) in searching for and synchronizing onto the Frame Head. In normal operation the modem will
only require servicing by the µC once per received or transmitted data block.
Thus, to transmit a block, the host µC has only to load the unformatted (raw) binary data into the modem's
data 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 with Forward Error Correction coding, interleave then scramble
the bits before transmission.
In receive mode, the modem can be instructed to assemble a block's worth of received bits, de-scramble and
de-interleave the bits, check and correct them (using the FEC coding) and check the resulting CRC before
placing the received binary data into the Data Buffer for the µC to read.
The modem can also handle the transmission and reception of unformatted data, to allow the transmission of
special Bit and Frame Synchronization sequences or test patterns.
4.3 Data Formats
4.3.1 General Purpose Formats
In a proprietary system the user may employ the data elements provided by this device to construct a custom,
over-air data structure.
For example, 16 bits of bit sync + 2 bytes of frame sync + 4 bytes of receiver and sender address + n data
blocks would be:
transmitted as:
TQB (bit and frame sync) + TQB (addresses) + (n x TDB) + TSB
received as:
SFS + RSB + RSB + RSB + RSB + (n x RDB)
Note: It is important to have established frame Synchronization before receiving data to enable the receiving
device to decode synchronously. The user may add, by way of algorithms performed on the controlling
device, additional data correction with the bytes in the data block task.
4.3.2 Mobitex¥ Frame Structure
The Mobitex¥ format for transmitted data is in the form of a Frame Head immediately followed by a number of
Data Blocks (0 to 32).
The Frame Head consists of 7 bytes:
2 bytes of bit sync:
1100110011001100 from base,
0011001100110011 from mobile
bits are transmitted from left to right
¤2001 MX-COM, Inc.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480134.005
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