Specifications

AN93
Rev. 0.8 7
Modem and DAA Operation
This section describes hardware design requirements
for optimum Si2493/57/34/15/04 modem chipset
implementation. There are three important
considerations for any hardware design. First, the
reference design and components listed in the
associated bill-of-materials should be followed exactly.
These designs reflect field experience with millions of
deployed units throughout the world and are optimized
for cost and performance. Any deviation from the
reference design schematic and components will likely
have an adverse affect on performance. Secondly,
circuit board layouts must rigorously follow "Appendix
A—ISOmodem® Layout Guidelines" on page 154.
Deviations from these layout techniques will likely
impact modem performance and regulatory compliance.
Finally, all reference designs use a standard component
numbering scheme. This simplifies documentation
references and communication with the Silicon
Laboratories technical support team. It is strongly
recommended that these same component reference
designators be used in all ISOmodem designs.
The following sections describe the operation and
design considerations of the modem chip, DAA chip,
and associated circuitry.
Modem (System-Side) Device
The Si2493/57/34/15/04 modem device contains a
controller, a DSP, program memory (ROM), data
memory (RAM), a serial and parallel interface, a crystal
oscillator, and an isolation capacitor interface. The
Figure 2 on page 8 clearly shows that in spite of the
significant internal complexity of the chip, the external
support circuitry is very simple. The following section
describes the function and use of the pins and some
important considerations for the selection and
placement of components.
Crystal Oscillator
The crystal oscillator circuit requires a 4.9152 MHz
fundamental mode parallel-resonant crystal. Typical
crystals require a 20 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 33 pF capacitor provides
40 pF per terminal, which, in series, yields the proper
20 pF load for the crystal.
Frequency stability and accuracy are critically important
to the performance of the modem. ITU-T specifications
require less than 200 ppm difference in the carrier
frequency 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 to within ±100 ppm. This tolerance includes
the initial accuracy of the crystal, frequency drift over
the temperature range the crystal will experience, and
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.
The CLKIN/XTALI pin (pin 1) can accept a 3.3 V
external 4.9152 MHz clock signal meeting the accuracy
and stability requirements described above. This is the
only input pin on the modem that is not 5 V tolerant. The
Si2493/57/34/04 will accept a 27 MHz clock that meets
the voltage and stability requirements described above.
Enabling this mode of operation is described in Table 26
on page 53.
The CLKOUT/A0 pin (pin 3) outputs a signal derived
from the 4.9152 clock. If the frequency of the output is
controlled via register U6E (CK1) using Si2404 or
Si2415, this signal is programmable from 2.64 MHz to
40.96 MHz. If using 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. If
R1 = 00000b, CLKOUT is disabled. If R1 = 11111b
(default), CLKOUT = 2.048 MHz.