Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 44)

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44 Rev. 1.3
The second mode is a 32.768 kHz fundamental mode parallel-resonant crystal. Typical crystals require a 12.5 pF
load capacitance. This load is calculated as the series combination of the capacitance from each crystal terminal to
ground, including parasitic capacitance due to package pins and PCB traces. The parasitic capacitance is
estimated as 7 pF per terminal. This, in combination with the 18 pF capacitors, provides 25 pF per terminal, which,
in series, yields the proper 12.5 pF load for the crystal.
Instead of a using a 32.768 kHz crystal, a signal at 32.768 kHz can be applied to the XTALI pin. In such a case, the
crystal loading caps should not be used.
The third mode is to use a 27 MHz clock signal. A crystal cannot be used for this mode, and the signal must be
applied to the XTALI pin.
Frequency stability and accuracy are critically important to the performance of the modem. ITU-T specifications
require less than 200 ppm difference between the carrier frequencies of two modems. This value, split between the
two modems, requires the oscillator frequency of each modem to be accurate and stable over all operating
conditions within ±100 ppm. This tolerance includes the initial accuracy of the crystal, the frequency drift over the
temperature range that the crystal is expected to experience, and the five-year aging of the crystal. Other factors
affecting the oscillator frequency include the tolerance and temperature drift of the load capacitor values. For
optimal V.92 performance, it is recommended to increase the oscillator stability to ±25 ppm.
For all the above three modes of operation, the CLKIN/XTALI pin (Pin 1) can accept a 3.3 V external clock signal
meeting the accuracy and stability requirements described above.
The CLKOUT/A0 pin outputs a signal derived from the 4.9152 MHz clock. If the frequency of the output is
controlled via register U6E (CK1) using the Si2404 or Si2415, this signal is programmable from 2.64 MHz to
40.96 MHz. If using the Si2434 or Si2457, this signal is programmable from 3.17 MHz to 49.152 MHz. There are
two special cases for the value of R1: R1 = 00000, CLKOUT is disabled, and R1 = 11111 (default),
CLKOUT = 2.048 MHz.
On older parts, the CLKOUT pulse starts immediately after RESET goes high, but, on the most recent versions
(those including SPI and 32 kHz operation), there is a small delay after RESET goes high. The delay is of
approximately 200 µs when using 4.91592 MHz or 27 MHz and approximately 8 ms when using a 32 kHz clock.
4.1.4. Ringer Network
R7 and R8 comprise the ringer network. These components determine the modem’s on-hook impedance at Tip
and Ring. These components are selected to present a high impedance to the line, and care must be taken to
ensure the circuit board area around these components is clean and free of contaminants, such as solder flux and
solder flakes. Leakage on RNG1 (Si3018/10 pin 8) and RNG2 (Si3018/10 pin 9) can impair modem performance.
R7 and R8 are also used by the modem to monitor the line voltage.
4.1.5. Optional Billing-Tone Filter
To operate without degradation during billing tones in Germany, Switzerland, and South Africa, an external LC
notch filter is required. (The Si3018/10 will remain off-hook during a billing tone event, but modem data may be lost,
or a modem disconnect or retrain may occur, 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 require billing-tone support, this filter is typically placed in an external dongle or added as a population
option. Figure 17 shows an example billing tone filter. L3 must carry the entire loop current. The series resistance
of the inductors is important to achieve a narrow and deep notch. This design has more than 25 dB of attenuation
at 12 kHz and 16 kHz. The billing tone filter degrades the ac termination and return loss slightly, but the global
complex ac termination passes worldwide return-loss specifications with and without the billing tone filter by at
least 3 dB.