CMX878D6 View Datasheet(PDF) - CML Microsystems Plc
Part Name
Description
MFG CO.
'CMX878D6' PDF : 63 Pages View PDF
Line Powered Modem plus DAA
CMX878
Figure 12b Filter Implementation
Programming is done by writing a sequence of twenty-seven 16-bit words to the Programming Register.
The first word should be 32769 (8001 hex), the following twenty-six 16-bit words set the frequencies and
levels and are in the range 0 to 32767 (0000-7FFF hex).
Word
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Value written
32769
Filter #1 coefficient b21
Filter #1 coefficient b11
Filter #1 coefficient b01
Filter #1 coefficient a21
Filter #1 coefficient a11
Filter #1 coefficient b22
Filter #1 coefficient b12
Filter #1 coefficient b02
Filter #1 coefficient a22
Filter #1 coefficient a12
Freq measurement #1 ncycles
Freq measurement #1 mintime
Freq measurement #1 maxtime
Word Value written
15 Filter #2 coefficient b21
16 Filter #2 coefficient b11
17 Filter #2 coefficient b01
18 Filter #2 coefficient a21
19 Filter #2 coefficient a11
20 Filter #2 coefficient b22
21 Filter #2 coefficient b12
22 Filter #2 coefficient b02
23 Filter #2 coefficient a22
24 Filter #2 coefficient a12
25 Freq measurement #2 ncycles
26 Freq measurement #2 mintime
27 Freq measurement #2 maxtime
The coefficients are entered as 15-bit signed (two’s complement) integer values (the most significant bit
of the 16-bit word entered should be zero) calculated as 8192 * coefficient value from the user’s filter
design program (i.e. this allows for filter design values of -1.9999 to +1.9999).
The design of the IIR filters should make allowance for the fixed receive signal filtering in the CMX878
which has a low pass characteristic above 1.5kHz of 0.4dB at 2kHz, 1.2dB at 2.5kHz, 2.6dB at 3kHz and
4.1dB at 3.4kHz.
‘ncycles’ is the number of signal cycles for the frequency measurement.
‘mintime’ is the smallest acceptable time for ncycles of the input signal expressed as the number of
9.6kHz timer clocks. i.e. ‘mintime’ = 9600 * ncycles / high frequency limit
‘maxtime’ is the highest acceptable time for ncycles of the input signal expressed as the number of
9.6kHz timer clocks. i.e. ‘maxtime’ = 9600 * ncycles / low frequency limit
The level detectors include hysteresis. The threshold levels - measured on the line with unity gain filters,
using the line interface circuit shown in Figure 4a, and with the Rx Gain Control block set to 0dB - are
nominally:
‘Off’ to ‘On’ -44.5dBm
‘On’ to ‘Off’ -47.0dBm
© 2002 Consumer Microcircuits Limited
46
D/878/2
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