CPC5604
www.clare.com
17
XXX
Ring Signal Detection
The snoop circuit actively monitors the telephone line
for 2 conditions:
1. Incoming ring signal
2. Caller ID information
The Snoop circuit “snoops” the line continuously while
the LiteLink
TM
is in the on-hook mode. Current taken
from the telephone line in the on-hook condition by the
LITELINK
TM
is maintained at a low 2uA maximum thus
meeting regulatory requirements for minimum on-hook
impedance limitation. When the central office places
the ring signal on the telephone line, that signal is cou-
pled through a pair of RC circuits to a differential ampli-
fier in the LiteLink
TM
.
Referring to Block Diagram, snoop capacitors connect-
ed to the SNP1/SNP2 pins provide a high voltage isola-
tion barrier between the host and the telephone line
while coupling the AC signals to the snoop amplifier.
The ring signal is digitized and brought out to the RING
pin where the host can qualify it as a valid ring signal.
The ring detection threshold is dependent on the values
of 3 external components: RRXF (R3), RSNOOP (R5 or
R6), and CS (C6 or C7). The default values in the typi-
cal bill of materials reflects the parameters in the data
sheet for typical operation. If it is desired to change the
threshold, the values can be selected by using the
equation:
Where f = ring frequency typically 20Hz.
= 330E-3
(R
SNOOP)
2
+ 1
5R
RXF
(2πf C
S
)
2
V
RING(PEAK)
Figure 2D Connection To Host Single Ended
(Transmit)
Care should be taken when using this equation since
RRXF (R3), CS (C6 or C7), and RSNOOP (R5 or R6)
affect receive gain and Caller ID gain. It is recom-
mended that RRXF (R3) be set to the typical value and
then after adjusting the ring detect threshold, check that
CID gain is acceptable.
Caller ID Detection
Caller ID (CID) is a service offered by the telephone
company to provide caller information (i.e. caller’s tele-
phone number) to the called party. CID service is
optional and signals only appear on the telephone lines
of subscribers that pay for this feature. The CID infor-
mation appears on the telephone line after the first ring
burst is sent from the central office (CO).
Some of the characteristics of the CID signal are sum-
marized below:
Parameter Value
Signal Level -13dBm
Link Type Simplex, 2W
Transmission Scheme Phase-coherent, FSK
Logical 1 (mark) 1200±12Hz
Logical 0 (Space) 2200± 22Hz
Transmission Rate 1200bps
Data serial binary async
BER < 10E -5
Bit Duration 833±50uS (same for start/stop as
well)
Full details about the CID signal can be found in Bellcore
document TR-TSY-000030, issue 1/1988.
Figure 2.7.A shows the CID timing diagram. Waveform
#1 represents the Analog signals on the telephone line
(amplitude not drawn to scale), waveform #2 is the dig-
ital RING detect output from the LiteLink
TM
, waveform
#3 is the CID input to the LiteLink
TM
from the Host. After
the first ring burst is detected by the host, the host
enables the CID line which automatically couples the
snoop circuit to the RX outputs on the LiteLink
TM
.
