FX919B Datasheet, PDF (27/47 Pages) CML Microcircuits – Excellent Radio Fading Performance

4-Level FSK Modem Data Pump
FX919B
Note: When the modem is in transmit mode and the Interleave Buffer is empty, a mid level (half-way between
'+1' and '-1') signal will be sent to the RRC filter.
In receive mode this bit will be '0'.
Status Register B4: DIBOVF - De-Interleave Buffer Overflow
In receive mode this bit will be set to '1' - also setting the IRQ bit - when a RHB, RILB or R4S task is written to
the Command Register too late to allow continuous reception.
The bit is cleared to '0' immediately after reading the Status Register, by writing a RESET task to the Command
Register or by changing the TXRXN or PSAVE bits of the Mode Register.
In transmit mode this bit will be '0'.
Status Register B3: CRCERR - CRC Checksum Error
In receive mode this bit will be updated at the end of a SFSH, RHB or RILB task to reflect the result of the
receive CRC check. '0' indicates that the CRC was received correctly, '1' indicates an error.
Note that this bit should be ignored when an 'Intermediate' block (which does not have an integral CRC) is
received.
The bit is cleared to '0' by a RESET task, or by changing the TXRXN or PSAVE bits of the Mode Register. In
transmit mode this bit is '0'.
Status Register B2, B1, B0
These bits are reserved for future use.
1.5.5.6 Data Quality Register
In receive mode, the FX919B continually measures the 'quality' of the received signal, by comparing the actual
received waveform over the previous 64 symbol times against an internally generated 'ideal' 4-level FSK
baseband signal.
The result is placed into bits 3-7 of the Data Quality Register for the µC to read at any time, bits 0-2 being
always set to '0'. Figure 14 shows how the value (0-255) read from the Data Quality Register varies with
received signal-to-noise ratio:
250
200
150
DQ
100
50
0
8
9
10
11
12
13
14
15
16
S/N dB (Noise in 2 x Symbol Rate Bandwidth)
Figure 14 Typical Data Quality Readingvs S/N
© 1997 Consumer Microcircuits Limited
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D/919B/1