73K222AL Datasheet, PDF (23/28 Pages) TDK Electronics – V.22, V.21, Bell 212A, 103 Single-Chip Modem

73K222AL
V.22, V.21, Bell 212A, 103
Single-Chip Modem
Unlike digital logic circuitry, modem designs must
properly contend with precise frequency tolerances
and very low level analog signals, to ensure
acceptable performance. Using good analog circuit
design practices will generally result in a sound
design. Following are additional recommendations
which should be taken into consideration when starting
new designs.
CRYSTAL OSCILLATOR
The K-Series crystal oscillator requires a parallel mode
(antiresonant) crystal which operates at 11.0592 MHz.
It is important that this frequency be maintained to
within ±0.01% accuracy.
In order for a parallel mode crystal to operate correctly
and to specification, it must have a load capacitor
connected to the junction of each of the crystal and
internal inverter connections, terminated to ground.
The values of these capacitors depend primarily on the
crystal’s characteristics and to a lesser degree on the
internal inverter circuit. The values used affect the
accuracy and start up characteristics of the oscillator.
LAYOUT CONSIDERATIONS
Good analog/digital design rules must be used to
control system noise in order to obtain highest
performance in modem designs. The more digital
circuitry present on the PC board, the more this
attention to noise control is needed. The modem
should be treated as a high impedance analog device.
A 22 µF electrolytic capacitor in parallel with a 0.1 µF
ceramic capacitor between VDD and GND is
recommended. Liberal use of ground planes and larger
traces on power and ground are also highly favored.
High speed digital circuits tend to generate a significant
amount of EMI (Electro-Magnetic Interference) which
must be minimized in order to meet regulatory agency
limitations. To accomplish this, high speed digital
devices should be locally bypassed, and the telephone
line interface and K-Series device should be located
close to each other near the area of the board where
the phone line connection is accessed. To avoid
problems, power supply and ground traces should be
routed separately to the analog and digital functions on
the board, and digital signals should not be routed near
low level or high impedance analog traces. The analog
and digital grounds should only connect at one point
near the K-Series device ground pin to avoid ground
loops. The K-Series modem IC’s should have both high
frequency and low frequency bypassing as close to the
package as possible.
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USING THE SERIAL MODE ON THE 73K222AL
A sensitivity to specific patterns being written to
the control registers in the 73K212/221/222AL and
73K222BL modem data pumps has been seen on
some parts when used in the serial control
interface mode. An alternating pattern followed by
its complement can cause the registers to not
have the intended data correctly written to the
registers. Specifically, if an alternating ..1010..
pattern is followed by its compliment, ..0101.., the
register may instead be programmed with a
..0001.. pattern. After analysis, it has been found
that any normal programming sequence should
not include these steps with one exception, and
that is in DTMF dialing. Since any random DTMF
sequence could be dialed, there is the potential for
these patterns to appear. For example, if a DTMF
digit “5” , 0101 bin is followed by a DTMF digit “0” ,
1010 bin, some parts will instead transmit a DTMF
digit “8”, 1000 bin, in its place. The solution to this
problem is to always clear the DTMF bits, D3-D0,
between dialed digits. This will not add additional
time to dialing since there is ample time between
digits when the DTMF bits can be cleared.
Previously during the DTMF off time the next digit
would be loaded into the TONE register. It is now
recommended to first clear bits D3-D0, then the
next digit to be dialed is loaded into the DTMF bits.
As mentioned earlier, under normal circumstances
these patterns would not be programmed for other
registers. If for some reason other registers are
programmed in such a way that an alternating
pattern is followed by its compliment, those bits
should be cleared before the complimentary
pattern is sent.
This method has been tested over the entire
voltage and temperature operating ranges. It has
been found to be a reliable procedure to ensure
the correct patterns are always programmed.
MODEM PERFORMANCE
CHARACTERISTICS
The curves presented here define modem IC
performance under a variety of line conditions
while inducing disturbances that are typical of
those encountered during data transmission on
public service telephone lines. Test data was
taken using an AEA Electronics’ “Autotest I”
modem test set and line simulator, operating
under computer control. All tests were run
full-duplex, using a Concord Data Systems 224 as
the reference modem. A 511 pseudo-random-bit