CMX969 Consumer Microcircuits Limited, CMX969 Datasheet - Page 12

• Manufacturer Part Number: CMX969
• Description: RD-LAP/MDC4800/Motient/ARDIS4-Level FSK Packet Data Modem
• Manufacturer: Consumer Microcircuits Limited

RD-LAP/MDC4800 Motient/ARDIS
1.5.5
The modem appears to the programmer as 4 write only 8-bit registers shadowed by 3 read only registers,
individual registers being selected by the A0 and A1 chip inputs: Note that there is a minimum allowable time
between accesses of the modem's registers, see Section 1.7.1 for details.
1.5.5.1
This is a 12-byte read/write buffer which is used to transfer data (as opposed to command, status, mode, data
quality or control information) between the modem and the host µC. It appears to the µC as a single 8-bit
register; the modem ensuring 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 C does not read the received data from the Data Buffer.
1.5.5.2
This 8-bit write-only register controls the modem's operating mode (RD-LAP or MDC), symbol rate and the
response times of the receive clock extraction and signal level measurement circuits.
Control Register B7, B6: CKDIV - Clock Division Ratio
These bits control a frequency divider driven from the clock signal present at the XTALN pin, and hence
determine the nominal symbol and bit rates. The following table shows the settings of B7 and B6 needed for
19200bps RD-LAP and 4800bps MDC4800 operation.
2001 Consumer Microcircuits Limited
The Programmer's View
B7
Data Block Buffer
Control Register
0
0
1
1
B6
0
1
0
1
A1
0
0
1
1
A0
Xtal Frequency
0
1
0
1
4.9152 MHz
7.3728 MHz
9.8304 MHz
See note
Command Register
Write to Modem
Control Register
Mode Register
Data Buffer
12
Xtal frequency / RD-LAP
Symbol Rate
Reserved for other uses.
1024
1536
Data Quality Register
512
768
Read from Modem
Status Register
Data Buffer
Division ratio:
MDC4800 Symbol Rate
Xtal frequency /
1024
1536
2048
3072
CMX969
D/969/5