Low Power V.22 bis Modem
CMX868
Programming is done by writing a sequence of up to seventeen 16-bit words to the Programming
Register.
The first word should be 32768 (8000 hex), the following 16-bit words set the frequencies and levels and
are in the range 0 to 16383 (0-3FFF hex)
Word
Tone Pair Value written
32768
1
2
3
4
5
6
7
- - -
- - -
16
17
TA
TA
TA
TA
TB
TB
- - -
- - -
TD
TD
Tone 1 frequency
Tone 1 level
Tone 2 frequency
Tone 2 level
Tone 1 frequency
Tone 1 level
- - - - - - - - - -
- - - - - - - - - -
Tone 2 frequency
Tone 2 level
The Frequency values to be entered are calculated from the formula:
Value to be entered = desired frequency (Hz) * 3.414
i.e. for 1kHz the value to be entered is 3414 (or 0D56 in Hex).
The Level values to be entered are calculated from the formula:
Value to be entered = desired Vrms * 93780 / VDD
i.e. for 0.5Vrms at VDD = 3.0V, the value to be entered is 15630 (3D0E in Hex)
Note that allowance should be made for the transmit signal filtering in the CMX868 which attenuates the
output signal for frequencies above 2kHz by 0.25dB at 2.5kHz, by 1dB at 3kHz and by 2.2dB at 3.4kHz.
On powerup or after a reset, the tone pairs TA-TC are set to notone, and TD set to generate 2130Hz +
2750Hz at approximately –20dBm each.
Receive Tone Pair Programming
The programmable tone pair detector is implemented as shown in Figure 12a. The filters are 4th order IIR
sections. The frequency detectors measure the time taken for a programmable number of complete input
signal cycles and compare this time against programmable upper and lower limits.
Figure 12a Programmable Tone Detectors
ã 2004 CML Microsystems Plc
33
D/868/9
CMLMICRO [ CML MICROCIRCUITS ]
