GMSK Packet Data Modem and RF Transceiver
CMX990
5.1.6
Application Notes
5.1.6.1 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, assuming the device is not starting from a powersave state, TXRXN
is set to ‘1’ and that the relevant control bits 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 stabilise, followed by setting T7H task.
2. Device interrupts host µC with IRQN 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 IRQN when 18th byte is read from the Data Buffer.
5. Status 1 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’ if another Data Block in this Frame is to be transmitted
GOTO ‘6’ 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 more data is to be sent then a new task need not
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 19.
Hang Byte
The filtering required to reduce the transmitted bandwidth causes energy from each bit of
information to be smeared 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’.
ã 2004 CML Microsystems Plc
40
D/990/1
CMLMICRO [ CML MICROCIRCUITS ]
