CMX878D6 View Datasheet(PDF) - CML Microsystems Plc
Part Name
Description
MFG CO.
'CMX878D6' PDF : 63 Pages View PDF
Line Powered Modem plus DAA
CMX878
1.6 Application Notes
1.6.1 Controlling the Phone Line
The CMX878 needs to control the phone line in various operating states. The main states are
summarised in the following table.
Mode of
Operation
On-hook,
min. current
On-hook,
Regulator
ON
Off-hook
taking line
current
(DC Mask)
Programmed
Line Current
Draw
Explanation
This is the On-hook or Standby state. The regulator is disabled and therefore
also the microcontroller. Only the Standby circuits remain active. The device
may have previously been primed for detection of a ring or line reversal event.
It will also respond to a WAKE pin event.
In this state the regulated supply is active but the gyrator / off-hook current
draw is disabled. There will be a current drawn from the line by the regulator,
providing a 3.3V for the CMX878, the microcontroller and any additional
application. The current taken will not be sufficient to take the line off-hook.
This is the off-hook state when the line is in-use. The regulator and gyrator are
enabled. Current will be drawn from the line to take the line off-hook. It will
also present the correctly matched impedance to AC signals. The CMX878 can
now be programmed to transmit and receive tones and modem signals. The
line voltage can be monitored by the ADC.
In this state the regulator is enabled and will draw a small line current (regulator
+ microcontroller + active CMX878 circuitry). An additional line current draw
can be set by controlling the level of the DAC. This feature can assist with
characterising the line. Whilst progressively increasing the line current,
measure the line voltage with the ADC and check for the presence of a Dial
Tone with the Call Progress Detector. When Dial Tone is detected, store a
representation of the line voltage in one of the Standby Supply registers. This
knowledge of the line voltage at below which the line is off-hook can be used to
determine if an extension has already seized the line.
The next table shows which circuits will need to be enabled.
Mode of
Operation
On-hook,
min. current
On-hook,
Regulator
ON
Off-hook on
DC Mask
Programmed
Line Current
Draw
Regulator
Enabled?
N
Y
Y
Y
Gyrator
Enabled?
N
N
Y
N
DAC
Enabled?
N
N
N
Y
ADC
Enabled?
N
Y if want to
measure line
voltage
Y to detect
an extension
going off-hook
Y
Note that in order to use any of the MODEM & TONES PROCESSOR functions of the CMX878, bit 7 of
the General Control Register ‘Pwr’ must also be set to 1 (otherwise the MODEM & TONES PROCESSOR
will be powersaved).
© 2002 Consumer Microcircuits Limited
48
D/878/2
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