CML Microcircuits CMX649, Примечание к применению

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CMX649 Wireless Voice Link Design Guide
©
2004 CML Microcircuits Page 16 of 42
Data is exchanged between the RF transceiver and the microcontroller in a
half-duplex fashion via the microcontroller’s SPI port. Data is transferred in a full-
duplex fashion, however, between the microcontroller and the ‘burst mode’ interface
of the CMX649.
A large difference exists between the CMX649 voice coding rate (27.8kbps) and the
over-the-air baud rate used by the MICRF505 (166.6kbps). If the RF transmitter and
CMX649 were activated at the same time, the RF transmitter would transmit the data
faster than the CMX649 could generate the data, and an underflow condition would
quickly occur. In a similar manner, the 166.6kbps received data from the transceiver
would quickly overwrite data being sent to the CMX649 at 27.8kbps. To manage the
speed mismatches and prevent data overwrites/underwrites, a buffering scheme is
implemented in the microcontroller.
Eighty bytes of RAM in the MSP430F1232 are used as data buffers for both transmit
(“RFTX_BUF”) and receive (“RFRX_BUF”) modes. Two microcontroller registers are
used as counters to track the flow of data in and out of RFTX_BUF. These counters,
called FROM649_CNTR and TORF_CNTR, are set to their maximum values when
the transmit mode is first started. The FROM649_CNTR counter is decremented
with each byte of encoded voice data that is passed from the CMX649 to the
MSP430F1232. As the RFTX_BUF fills up, the FROM649_CNTR decreases in
value. When the FROM649_CNTR decreases to the correct value, configuration
data is loaded to the MICRF505 and the RF transmitter is enabled.
1 80
Data input from
CMX649
Data output to
MICRF505
79 34
RF transmission
begins
RFTX_BUF byte location
Manchester
Encoding
FROM649_CNTR
at maximum value
FROM649_CNTR
at minimum value
Figure 5, RFTX_BUF Byte Locations
After the RF transceiver is configured and enabled, the following header information
is passed from the microcontroller to the transceiver:
•
Bit sync (12 bytes)
•
Frame sync (2 bytes)
•
Command information (2 bytes)
•
Identification information (2 bytes)
The voice data and header information is Manchester encoded in the microcontroller;
this is done to add sufficient transitions to the data signal to prevent the PLL from