SI2414 SILABS [Silicon Laboratories], SI2414 Datasheet - Page 69

• Manufacturer Part Number: SI2414
• Manufacturer: SILABS [Silicon Laboratories]

AC Termination
The Si2456/33/14 has two ac Termination impedances
that are selected with the ACT bit (U67, bit 5).
ACT = 0 is a real nominal 600 Ω termination, which
satisfies the impedance requirements of FCC part 68,
JATE, and other countries. This real impedance is set
by circuitry internal to the Si3015 as well as the resistor
R2 connected to the REXT pin.
ACT = 1 is a complex impedance, which satisfies the
impedance requirements of Australia, New Zealand,
South Africa, CTR21, and some European NET4
countries, such as the UK and Germany. This complex
impedance is set by circuitry internal to the Si3015 as
well as the complex network formed by R12, R13, and
C14 connected to the REXT2 pin.
Ringer Impedance
The ring detector in a typical DAA is ac-coupled to the
line with a large 1 µ F, 250 V decoupling capacitor. The
ring detector on the Si2456/33/14 is also capacitively-
coupled to the line, but it is designed to use smaller, less
expensive 1.8 nF capacitors. Inherently, this network
produces a high ringer impedance to the line of
approximately 800–900 k Ω . This value is acceptable for
most countries including FCC and CTR21.
Several countries including the Czech Republic,
Poland, South Africa, and South Korea require a
maximum ringer impedance. For Poland, South Africa,
and South Korea, the maximum ringer impedance
specification can be met with an internally-synthesized
impedance by setting the RZ bit (U67, bit 1).
Pulse Dialing
Pulse dialing is accomplished by going off- and on-hook
to generate make and break pulses. The nominal rate is
10 pulses per second. Some countries have very tight
specifications for pulse fidelity including make and
break times, make resistance, and rise and fall times. In
a traditional solid-state dc holding circuit, there are a
number of issues in meeting these requirements.
The Si2456/33/14 dc holding circuit has active control of
the on-hook and off-hook transients to maintain pulse
dialing fidelity.
Spark quenching requirements in countries, such as
Italy, the Netherlands, South Africa, and Australia deal
with the on-hook transition during pulse dialing. These
tests provide an inductive dc feed resulting in a large
voltage spike. This spike is caused by the line
inductance and the sudden decrease in current through
the loop when going on-hook. The traditional way of
dealing with this problem is to put a parallel RC shunt
across the hookswitch relay. The capacitor is large
Rev. 0.9
(~1 µ F, 250 V) and expensive. In the Si2456/33/14, the
OHS bit (U67, bit 6) can be used to slowly ramp down
the loop current to pass these tests without requiring
additional components.
DTMF Dialing
In CTR21 dc termination mode, the DIAL bit (U69, bit 6)
should be set during DTMF dialing if LVCS ≤ 12. Setting
this bit increases headroom for large signals. This bit
should only be used during dialing and if LVCS < 11.
In
bits 3:2] = 01), the ISOmodem attenuates the transmit
output by 1.7 dB to meet headroom requirements.
Similarly, in Low Voltage mode (DCT = 00), the
ISOmodem attenuates the transmit output by 4 dB.
However, when DTMF dialing is desired in these
modes, this attenuation must be removed. This is
achieved by entering the FCC dc termination mode and
setting the FJM bit (U69, bit 5). When in the FCC dc
termination modes, these bits enable the respective
lower loop current termination modes without the
associated transmit attenuation. Increased distortion
may be observed, which is acceptable during DTMF
dialing. After DTMF dialing is complete, the attenuation
should be enabled by returning to either the Japan dc
termination mode (DCT = 01) or the Low Voltage
termination mode (DCT = 00). The FJM and the FLVM
bits have no effect in any other termination mode other
than the FCC dc termination mode.
Billing Tone Detection
“Billing tones” or “metering pulses” generated by the
central office can cause modem connection difficulties.
The billing tone is typically a 12 kHz or 16 kHz signal
and is sometimes used in Germany, Switzerland, and
South Africa. Depending on line conditions, the billing
tone may be large enough to cause major modem
errors. The Si2456/33/14 chipset can provide feedback
when a billing tone occurs and when it ends.
Billing tone detection is enabled by setting the BTE bit
(U68, bit 2). Billing tones less than 1.1 V
are filtered out by the low-pass digital filter on the
Si2456/33/14. The ROV bit (U68, bit 1) is set when a
line signal is greater than 1.1 V
overload condition. The BTD bit is set when a line signal
(billing tone) is large enough to excessively reduce the
line-derived power supply of the line-side device
(Si3015). When the BTD bit is set, the dc termination is
changed to an 800 Ω dc impedance. This ensures
minimum line voltage levels even in the presence of
billing tones.
The OVL bit should be polled following billing tone
detection. When the OVL bit returns to 0, indicating that
Japan
Si2456/Si2433/Si2414
dc
termination
PK
mode
indicating a receive
PK
(DCT [U67,
on the line
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