FSK Modem and DTMF Codec
CMX865
Tx Mode Register b7-5: DTMF Twist (Tx DTMF Mode)
These three bits allow for adjustment of the DTMF twist to compensate for the frequency
response of different external circuits. The device varies the twist by making changes to the upper
tone-group levels. Note that the twist cannot be adjusted mid-tone.
b5
b7
b6
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
+2.0dB twist (normal setting when external response is flat)
+1.0dB twist
+1.5dB twist
+2.5dB twist (do not use in conjunction with the 0dB Tx level setting)
+3.0dB twist (do not use in conjunction with the 0dB Tx level setting)
+3.5dB twist (do not use in conjunction with the 0dB Tx level setting)
+4.0dB twist (do not use in conjunction with the 0dB Tx level setting)
+4.5dB twist (do not use in conjunction with the 0dB Tx level setting)
Tx Mode Register b4-3: Tx Data Format (Tx Modem Modes)
These two bits select Synchronous or Start-stop mode and the addition of a parity bit to
transmitted characters in the Start-stop mode.
b4
b3
1
1
0
0
1
0
1
0
Tx Synchronous mode
Tx Start-stop mode, no parity
Tx Start-stop mode, even parity bit added to data bits
Tx Start-stop mode, odd parity bit added to data bits
Tx Mode Register b2-0: Tx Data and Stop Bits (Tx Start-Stop Modes)
In Tx Start-stop mode these three bits select the number of Tx data and stop bits.
b2
b1
b0
1
1
1
1
0
0
0
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
8 data bits, 2 stop bits
8 data bits, 1 stop bit
7 data bits, 2 stop bits
7 data bits, 1 stop bit
6 data bits, 2 stop bits
6 data bits, 1 stop bit
5 data bits, 2 stop bits
5 data bits, 1 stop bit
Tx Mode Register b2-0: Tx Data Source (Tx Synchronous Mode)
In Tx Synchronous mode (b4-3 = 11) these three bits select the source of the data fed to the Tx
FSK modulator.
b2
b1
b0
1
0
0
0
x
1
1
0
x
1
0
x
Data bytes from Tx Data Buffer
Continuous 1s
Continuous 0s
Continuous alternating 1s and 0s
© 2005 CML Microsystems Plc
21
D/865/3
CMLMICRO [ CML MICROCIRCUITS ]
