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CMX635

ISDN Subscriber Processor

CML Microsystems Plc 

ISDN Subscriber Processor

CMX635

The FSK tones are automatically generated when the FSK function is enabled and FSK data is

written to the transmit uart. The tone frequencies for FSK transmission can be selected to be

compliant with either Bell 202 or V.23 standards.

The FSK UART can be programmed to convert the transmit data from octets to serial

asynchronous data characters by adding Start and Stop bits or to transmit the octets

synchronously with no start or stop bits. The baud rate is fixed at 1200 baud. Status flags indicate

when the transmit buffer requires more data and whether the buffer data has underflowed (the

UART has started a new FSK character before the buffer has been refreshed).

The DTMF decoder can accept a wide range of signal input amplitudes and produces a 4 bit

DTMF character code that can be read from the ‘DTMF Receive Data’ register. Status flags

indicate when a valid tone is being detected and when it ceases.

A Subscriber Pulse Metering (SPM) tone is available at a separate pin and can be programmed to

12kHz or 16kHz. When turned on, the output ramps from zero to full amplitude in approximately

4mS and ramps down again when turned off. This output allows emulation of the exchange call

charging information for POTS style payphones.

1.5.6 The Channel Routing Block

The Channel Routing block enables the active data channels (B1, B2, D, IC1, IC2) to be flexibly

routed between the ST interface, the IOM interface, the HDLC controllers and the Codec.

The available routing resources allow implementation of a full suite of loopback paths in either the

transparent (the incoming data continues to be passed to the original destination as well as

looping back) or the non-transparent (original source/destination for data is disabled) modes.

For B data routing, 4 prioritised routing registers are available. Each register contains a required

4-bit source and 4-bit destination port code. If there are conflicts between routing register data

sources, the highest priority register routing will be implemented and the lower priority routing will

be used as a destination only.

D-channel routing is contained within a single register, which is divided into high and low priority

routing 4-bit nibbles. Within each nibble 2-bit source and destination port codes can be

programmed. The priority mechanism works in a similar fashion to the B data routing above.

1.5.7

Speaker Phone Functions

The CMX635 contains comprehensive hardware filters, voice-above-noise detectors and signal

path attenuators to allow a sophisticated implementation of software controlled speakerphone

algorithms. The data processing is carried out on the 8kHz 8-bit wide PCM data in both the

receive and transmit paths. Status bits are available to indicate the presence of voice above the

background noise in both the receive and transmit channels and the amplitude of the detected

voice can be read by software. The software algorithm can then decide which is the dominant

channel and distribute the loop gain between receive and transmit channels.

The following data processing functions on both the receive and transmit digital data paths are

provided. Note that all data processing is done on companded data in both channels.

• Full wave rectification conversion of input data

• Noise filtering of converted data with long time constant (~ 10 s)

• Speech filtering of converted data with fast attack time constant (~ 1 ms) and selectable

slow decay time constant (128, 256, 500 ms).

• Auto Gated Noise Filter i.e. the statistics integration is suspended when speech is

detected in the channel.

• Interrupt control system based upon filter output comparators, which provides three

interrupt sources for each channel. The interrupt sources are: -

© 2001 Consumer Microcircuits Limited

20

D/635/2

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