CPC5604
Introduction
the host asserts the CID line which automatically cou-
ples the snoop circuit to the RX outputs on the
LITELINKTM. After the CID signal is processed by the
host, the host will deactivate the CID signal. At this point
the host can answer the call by asserting the OH signal.
Note that when the LITELINKTM goes off-hook it auto-
matically disconnects the snoop path from both RX and
RING outputs. Signals appearing on the telephone line
are now coupled through the optical isolation barrier in
the LITELINKTM and not via the snoop path.
The LITELINKTM (CPC5604) is a single package
International Data Access Arrangement solution that is
designed to be used in a variety of telephone applica-
tions including high performance 56kbps (V.90)
modems. The LITELINKTM uses advanced optical signal
coupling techniques to provide the required electrical
isolation between the telephone and the Customer
Premises Equipment (CPE). The LITELINKTM differs
from other solutions using optical or capacitive isolation
techniques by including the barrier inside the IC pack-
age, thus eliminating the need for external optocouplers
or high-voltage capacitors in the data path resulting in
overall reduced board space. The LITELINKTM has
been designed to meet or exceed the requirements of
international regulatory agencies.
Hook Switch Control
The OH or off-hook input is used to place the DAA on or
off-hook. When the input is High, the DAA is on-hook or
ready to receive calls from the CO. In this mode the
snoop circuitry is enabled as described above. Driving
OH Low places the DAA off-hook allowing the CO sup-
plied loop current to flow (120mA max.), indicating the
DAA is answering or preparing to place a call.
For international PTT compliance external passive com-
ponents can be changed to meet different country
requirements.
For added flexibility, a second device, the CPC5601,
can be used in conjunction with the CPC5604 to offer a
host programmable solution. The CPC5601 is pro-
grammed serially through the host’s microcontroller.
Using the CPC5601 along with the CPC5604 eliminates
the need to change external passive components allow-
ing for a flexible, fully international DAA.
Transmit Signal
Outgoing analog signals to be transmitted to the tele-
phone lines are placed differentially on the TX+ and TX-
inputs of the CPC5604. Transmit level from the user
device is limited to 0dBm or 2.1Vp-p. The differential
transmit signal is converted to a single ended signal by
the CPC5604. The transmit signal is transferred across
an optical barrier by an electrical-optical-electrical
amplifier, which is transparent to the user. Variations in
gain due to electrical-optical-electrical efficiency are vir-
tually eliminated by an on-chip automatic gain control
circuit which sets the input to output gain of the photo-
diode amplifier to 1. This results in a TX insertion loss of
+/- 1dB.
Ring Detection via Snoop Circuit
While in the on-hook state (OH deasserted), an internal
multiplexer turns on a “snoop” circuit that actively moni-
tors the phone line for two conditions: incoming ring sig-
nal and Caller ID (CID) information. The snoop circuit
“snoops” the line continuously while drawing a low 2uA
max. current from the telephone line thus meeting regu-
latory requirements. When the central office (CO) places
a ring signal on the telephone line, 90VRMS max, the
RING output is pulsed from High to Low for 2 seconds at
the same frequency as the AC signal, typically 20Hz, and
restored to High during the 4 second delay. The ring
detection circuitry is designed to reject false signaling
from pulse dialing circuits or noise on the line.
Caller ID (CID) Detection
via Snoop Circuit
CID is a service offered by the telephone company to
provide caller information (i.e. the caller’s telephone
number) to the called party. The CID signal is present on
the telephone line after the first ring burst is sent from
the CO. After this first ring burst is detected by the host,
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