Si3056
Si3018/19/10
enough to excessively reduce the line-derived power
5.17. Pulse Dialing and Spark Quenching
supply of the line-side device.
Pulse dialing results from going off- and on-hook to
generate make and break pulses. The nominal rate is
10 pulses per second. Some countries have strict
specifications for pulse fidelity that include make and
break times, make resistance, and rise and fall times. In
a traditional solid-state dc holding circuit, there are
many problems in meeting these requirements.
The OVL bit (Register 19) can be polled following a
billing tone detection. The OVL bit indicates that the
billing tone has passed when it returns to 0. The BTD
and ROV bits are sticky, and must be written to 0 to be
reset. After the billing tone passes, the DAA initiates an
auto-calibration sequence that must complete before
data can be transmitted or received.
Certain line events, such as an off-hook event on a
parallel phone or a polarity reversal, can trigger the ROV
or the BTD bits. Look for multiple events before
qualifying if billing tones are present. After the billing
tone passes, the DAA initiates an auto-calibration
sequence that must complete before data can be
transmitted or received.
Although the DAA remains off-hook during a billing tone
event, the received data from the line is corrupted when
a large billing tone occurs. If the user wishes to receive
data through a billing tone, an external LC filter must be
added. A manufacturer can provide this filter to users in
the form of a dongle that connects on the phone line
before the DAA. This prevents the manufacturer from
having to include a costly LC filter to support multiple
countries and customers.
The Si3056 dc holding circuit actively controls 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 solution to
the problem is to put a parallel resistive capacitor (RC)
shunt across the hookswitch relay. However, the
capacitor required is large (~1 µF, 250 V) and relatively
expensive. In the Si3056, loop current can be controlled
to achieve three distinct on-hook speeds to pass spark
quenching tests without additional BOM components.
Through settings of four bits in three registers, OHS
(Register 16), OHS2 (Register 31), SQ1 and SQ0 Alternatively, when a billing tone is detected, the system
(Register 59), a slow ramp down of loop current can be software notifies the user that a billing tone has
achieved which induces a delay between the time OH occurred. Notification prompts the user to contact the
bit is cleared and the time the DAA actually goes on- telephone company to disable billing tones or to
hook.
purchase an external LC filter.
Disturbance on the line other than billing tones can also
cause a receive overload. Some conditions may result in
a loop current collapse to a level below the minimum
required operating current of the DAA. When this occurs,
the dropout detect bit (DOD) is set, and an interrupt will
be generated if the dropout detect interrupt mask bit
(DODM) is set.
To ensure proper operation of the DAA during pulse
dialing, disable the automatic resistor calibration that is
performed each time the DAA enters the off-hook state
by setting the RCALD bit (Register 25, bit 5).
5.18. Billing Tone Protection and Receive
Overload
“Billing tones” or “metering pulses” generated by the
Central Office can cause modem connection difficulties.
The billing tone is typically either a 12 or 16 kHz signal
and is sometimes used in Germany, Switzerland, and
South Africa. Depending on line conditions, the billing
tone might be large enough to cause major errors in the
line data. The Si3056 chipset can provide feedback
indicating the beginning and end of a billing tone.
5.19. Billing Tone Filter (Optional)
To operate without degradation during billing tones in
Germany, Switzerland, and South Africa, requires an
external LC notch filter. The Si3056 can remain off-hook
during a billing tone event, but line data is lost in the
presence of large billing tone signals. The notch filter
design requires two notches, one at 12 kHz and one at
16 kHz. Because these components are expensive and
few countries utilize billing tones, this filter is typically
placed in an external dongle or added as a population
option for these countries. Figure 22 shows an example
billing tone filter.
Billing tone detection is enabled with the BTE bit
(Register 17, bit 2). Billing tones less than 1.1 V
on
PK
the line are filtered out by the low pass digital filter on
the Si3056. The ROV bit is set when a line signal is
greater than 1.1 V , indicating a receive overload
PK
condition. The BTD bit is set when a billing tone is large
Rev. 1.05
31
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