CMX867D2 CML Microcircuits, CMX867D2 Datasheet - Page 15

• Manufacturer Part Number: CMX867D2
• Description: Low power V.22 modem
• Manufacturer: CML Microcircuits

Low Power V.22 Modem
The FSK demodulator recognises individual frequencies as representing received ‘1’ or ‘0’ data bits:
The DPSK demodulator decodes DPSK modulation of a 1200Hz or 2400Hz carrier and is used for V.22
and Bell 212A modes. It includes an adaptive receive signal equaliser (auto-equaliser) that will
automatically compensate for a wide range of line conditions in DPSK modes. The auto-equaliser can
provide a useful improvement in performance in 600 or 1200bps DPSK modes, so although it must be
disabled at the start of a handshake sequence, it can be enabled as soon as scrambled 1200bps 1s have
been detected.
Both FSK and DPSK demodulators produce a serial data bit stream which is fed to the Rx pattern
detector, descrambler and USART block, see Figure 7a. The demodulator input is also monitored for
continuous dibits '00,11' in 1200bps DPSK mode and continuous alternating 1s and 0s in all other modes.
The DPSK demodulator also estimates the received bit error rate by comparing the actual received signal
against an ideal waveform. This estimate is placed in bits 2-0 of the Status Register, see Figure 10.
1.5.8
See Figure 7a.
The 1010.. pattern detector operates only in FSK modes and will set bit 9 of the Status Register when 32
bits of alternating 1’s and 0’s have been received.
The ‘Continuous Unscrambled 1’s’ detector operates in all modem modes and sets bits 8 and 7 of the
Status Register to ‘01’ when 32 consecutive 1’s have been received.
The descrambler operates only in DPSK modes and is enabled by setting bit 7 of the Rx Mode Register.
The ‘Continuous Scrambled 1’s’ detector operates only in DPSK modes when the descrambler is enabled
and sets bits 8 and 7 of the Status Register to ‘11’ when 32 consecutive 1’s appear at the output of the
descrambler. To avoid possible ambiguity, the ‘Scrambled 1’s’ detector is disabled when continuous
unscrambled 1’s are detected.
The ‘Continuous 0’s’ detector sets bits 8 and 7 of the Status Register to ‘10’ when NX consecutive 0’s
have been received, NX being 32 except when DPSK Start-Stop mode has been selected, in which case
NX = 2N + 4 where N is the number of bits per character including the Start, Stop and any Parity bits.
All of these pattern detectors will hold the ‘detect’ output for 12 bit times after the end of the detected
pattern unless the received bit rate or operating mode is changed, in which case the detectors are reset
within 2 msec.
1.5.9
A flexible Rx USART is provided for all modem modes, meeting the requirements of V.14 for DPSK
modems. It can be programmed to treat the received data bit stream as Synchronous data or as Start-
Stop characters.
In Synchronous mode the received data bits are all fed into the Rx Data Buffer which is copied into the
C-BUS Rx Data Register after every 8 bits.
In Start-Stop mode the USART Control logic looks for the start of each character, then feeds only the
required number of data bits (not parity) into the Rx Data Buffer. The parity bit (if used) and the presence
of a Stop bit are then checked and the data bits in the Rx Data Buffer copied to the C-BUS Rx Data
Register.
2001 Consumer Microcircuits Limited
Rx Modem Pattern Detectors and Descrambler
Rx Data Register and USART
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CMX867
D/867/2