FX929B Datasheet, PDF (15/48 Pages) CML Microcircuits – Flexible Operating Modes

4-Level FSK Modem Data Pump
FX929B
Command Register B4: TXIMP
This bit allows the user to choose between two transmit symbol shapes as described in section 1.5.7. Note
that this bit must be set correctly every time the Command Register is written to.
Command Register B3
This bit should always be set to '0'.
Command Register B2, B1, B0: TASK
Operations such as transmitting or receiving a data block are treated by the modem as 'tasks' and are initiated
when the µC writes a byte to the Command Register with the TASK bits set to anything other than the 'NULL'
code.
The µC should not write a task (other than NULL or RESET) to the Command Register or write to or read from
the Data Buffer when the BFREE (Buffer Free) bit of the Status Register is '0'.
Different tasks apply in receive and transmit modes.
When the modem is in transmit mode, all tasks other than NULL or RESET instruct the modem to transmit
data from the Data Buffer, formatting it as required. The µC should therefore wait until the BFREE (Buffer
Free) bit of the Status Register is '1', before writing the data to the Data Block Buffer, then it should write the
desired task to the Command Register. If more than 1 byte needs to be written to the Data Block Buffer, byte
number 0 of the block should be written first.
Once the byte containing the desired task has been written to the Command Register, the modem will:
Set the BFREE (Buffer Free) bit of the Status Register to '0'.
Take the data from the Data Block Buffer as quickly as it can - transferring it to the Interleave Buffer
for eventual transmission. This operation will start immediately if the modem is 'idle' (i.e. not
transmitting data from a previous task), otherwise it will be delayed until there is sufficient room in the
Interleave Buffer.
Once all of the data has been transferred from the Data Block Buffer the modem will set the BFREE
and IRQ bits of the Status Register to '1', (causing the chip IRQN output to go low if the IRQNEN bit of
the Mode Register has been set to '1') to tell the µC that it may write new data and the next task to the
modem.
This lets the µC write a task and the associated data to the modem while the modem is still transmitting the
data from the previous task.
Data from µC to Block Buffer
Task from µC to Command Register
BFREE bit of Status Register
Task 1
Task 2
IRQ bit of Status Register
IRQN o/p (IRQNEN = '1')
TXOP signal
from task 1
from task 2
Figure 8 Transmit Task Overlapping
© 1997 Consumer Microcircuits Limited
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D/929B/1