GMSK Modem Data Pump
Page 24 of 37
MX909A PRELIMINARY INFORMATION
5 Application
5.1 Transmit Frame Example
If the device is required to send a Mobitex¥ Frame the following control signals and data should be issued to
the modem, provided the device is not starting from a powersave state, TX/RX is set to '1' and that SCREN,
DARA, CKDIV, and DQEN have been set as required after power was applied to the device:
1. 6 bytes forming the Frame Head are loaded into the Data Buffer, followed by a 2-bit pause to let the filter
stabilize, followed by setting T7H task.
2. Device interrupts host µC with IRQ when the 6th byte is read from the Data Buffer.
3. Status Register is read and 18 bytes are loaded, followed by setting TDB task.
4. Device interrupts host µC with IRQ when 18th byte is read from the Data Buffer.
5. Status Register is read, host may load data and set next task as required:
GOTO '1'
if the last Data Block for this Frame has been transmitted and another Frame is to be
immediately transmitted
GOTO '3'
GOTO '6'
if another Data Block in this Frame is to be transmitted
if no more data is to be immediately sent
6. 1 byte representing the 'hang byte' is loaded into the Data Buffer, followed by setting the TSB task.
If the 'hang byte' has been transmitted and no further data is to be sent, then a new task does not need be
written and the µC can wait for the IBEMPTY interrupt ,when after a few bits, to allow for the Tx filter delay, it
can shut down the Tx RF circuits.
A top level flowchart of the transmit process is shown in Figure 14.
5.1.1 Hang Byte
The filtering required to reduce the transmitted bandwidth causes energy from each bit of information to be
spread across 3 bit times. To ensure that the last bit transmitted is received correctly it is necessary to add
an 8-bit 'hang byte' to the end of each message. Thus the tasks required to transmit an isolated Mobitex¥
frame are:
T7H + (n x TDB) + TSB
When receiving this data, the extra byte can be ignored as its only function is to ensure integrity of the last bit
and not to carry any information itself.
It is suggested that a '00110011' or '11001100' pattern is used for this 'hang byte'.
¤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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