User's Manual
4-Level FSK Modem Data Pump Page 15 of 47 MX919B PRELIMINARY INFORMATION
©2001 MX•COM, INC. www.mxcom.com Tele: 800 638 5577 336 744 5050 Fax: 336 744 5054 Doc. # 20480170.003
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SYMBOL
SYNC
SYMBOL
SYNC
SYMBOL
SYNC
FRAME
SYNC
FRAME
SYNC
FRAME
SYNC
'LAST'
BLOCK
'HEADER' BLOCKS
'INTERMEDIATE' BLOCKS
'INTERMEDIATE' BLOCKS
A
B
C
Figure 8: Alternative Frame Structures
The MX919B performs the entire block formatting and de-formatting required to convert data between µC
binary form and Over-Air as shown in Figure 7.
4.5 The Programmer’s View
To the programmer, the modem appears as 4 write only 8-bit registers, shadowed by 3 read only registers.
The individual registers are selected by the A0 and A1 chip inputs:
A1 A0 Write to Modem Read from Modem
0 0 Data Buffer Data Buffer
0 1 Command Register Status Register
1 0 Control Register Data Quality Register
1 1 Mode Register Not used
Note: There is a minimum time allowance between accesses of the modem's registers, see Section 6.1.4.
4.5.1 Data Block Buffer
This is a 12-byte read/write buffer used to transfer data (as opposed to command, status, mode, data quality
or control information) between the modem and the host µC.
To the µC, the Data Block Buffer appears as a single 8-bit register. The modem ensures that sequential µC
reads or writes to the buffer are routed to the correct locations within the buffer.
The µC should only access this buffer when the Status Register BFREE (Buffer Free) bit is '1'.
The buffer should only be written to while in Tx mode and read from while in Rx mode. Note that in receive
mode, the modem will function correctly even if the received data is not read from the Data Buffer by the µC.
4.5.2 Command Register
Writing to this register tells the modem to perform a specific task as indicated by the TASK bits and modified
by the AQSC, AQLEV, CRC, and TXIMP bits.
76543210
Command Register
AQSC TXIMP
Reserved
Set to '0'
TASK
AQLEV CRC
When there is no action to perform, the modem will be in an 'idle' state. If the modem is in transmit mode, the
input to the Tx RRC filter will be connected to V
BIAS
. In receive mode, the modem will continue to measure
the received data quality and extract symbols from the received signal, supplying them to the de-interleave
buffer, otherwise these received symbols are ignored.