FSK Modem and DTMF Codec
CMX865
5.10.8 Programming Register
Programming Register : 16-bit write-only.
C-BUS address $E8
This register is used to program the transmit and receive programmed tone pairs by writing appropriate
values to RAM locations within the CMX865. Note that these RAM locations are cleared by Powersave or
Reset.
The Programming Register should only be written to when the Programming Flag bit (b13) of the Status
Register is 1. The act of writing to the Programming Register clears the Programming Flag bit. When the
programming action has been completed (normally within 150µs) the CMX865 will set the bit back to 1.
When programming Transmit or Receive Tone Pairs, do not change the Transmit or Receive Mode
Registers until programming is complete and the Programming Flag bit has returned to 1.
Transmit Tone Pair Programming
4 transmit tone pairs (TA to TD) can be programmed.
The frequency (max 3.4kHz) and level must be entered for each tone to be used.
Single tones are programmed by setting both level and frequency values to zero for one of the pair.
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 = 3.3V, the value to be entered is 14209 (3781 in Hex)
Note that allowance should be made for the transmit signal filtering in the CMX865 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.
© 2005 CML Microsystems Plc
29
D/865/3
CMLMICRO [ CML MICROCIRCUITS ]
