Specifications

AN93
80 Rev. 0.8
U66 (DAA Control Register 5, DAAC5)
U66 (DAAC5) is a bit-mapped register with all bits
except bit 6 reserved. (See Table 53.)
Bit 6 (FDT) is a read-only bit that reports whether or not
an ISOcap™ frame lock is established. FDT is typically
used for board-level debugging and is not used during
normal modem operation.
U66 resets to 0x0040 with a power-on or manual reset
assuming framelock is established.
U67–U6A (International Configuration Registers)
International Configuration Registers include U67
through U6A. These are bit-mapped registers that
control international configuration settings, such as dc
and ac termination, ringer impedance and detection,
current limit, and billing tone protection.
U67 (ITC1)
U67 is a bit-mapped register with bits 5:4, 8, 11:10, and
15:14 reserved. (See Table 54.) U67 resets to 0x0008
with a power-on or manual reset.
Bit 7 (DCR) is used to set the dc line termination of the
modem. DCR = 0
b
is the normal mode of operation with
dc impedance selected by U67[3:2] (DCV). When
DCR = 1
b
, the device presents a dc line impedance of
800 Ω, which can be used to enhance operation with a
parallel phone, for improved low line voltage
performance and for overload. This bit must be set to 0
when the modem is on-hook. See "DC Termination" on
page 126 for details.
Bit 6 (OHS) is used to control the speed with which the
modem drops the line. The default setting, OHS = 0
b
,
causes the modem to go from the off-hook state
(drawing loop current) to the on-hook state (not drawing
loop current) quickly.
This operation is acceptable in many countries.
However, some countries, such as Italy, South Africa,
and Australia, have spark quenching requirements.
Spark quenching can be accomplished by placing a
resistor and a capacitor across the hookswitch or by
controlling the off-hook to on-hook transition speed to
prevent excessive voltage build-up. Slowly reducing the
loop current to zero fulfills the spark quenching
requirement without the extra components. Setting
OHS = 1
b
causes the hookswitch to turn off the loop
current with a ramp instead of a step.
Bits 3:2 (DCV) select the dc termination for the modem.
DCV = 00
b
is the lowest voltage mode supported on the
Si2493/57/34/15/04. DCV = 01
b
is the next lowest
voltage mode. See "DC Termination" on page 126 for
details.
Bit 1 (RZ) = 0 (default) allows ringer impedance to be
determined by external components. This impedance is
typically 800–900 kΩ. RZ = 1 enables on-chip synthesis
of a lower ringer impedance for countries, such as
Poland, South Africa, and South Korea.
Bit 0 (RT), Ring Threshold, is used to satisfy various
country ring detect requirements. RT = 0
b
(default) sets
the ring threshold for 11–22 V
RMS.
RT = 1
b
sets the ring
threshold for 17–33 V
RMS
. Signals below the lower level
of the range are not detected. Signals above the upper
level of the range are always detected.
Table 53. U66 Bit Map
Bit Name Function
15:7 Reserved Read returns zero.
6FDTFrame Detect.
0 = ISOcap frame lock not established
1 = ISOcap frame lock established
5:0 Reserved Read returns zero.