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SiI 141B SiI-DS-0037-C
Silicon Image, Inc. 9 Subject to Change without Notice
In some application, SCDT is connected to the PDO pin to provide a power savings mode. In others, SCDT is
connected to an external circuit to signal when an incoming video signal is available. These external devices may
use an internal pull up which can cause problems.
If SCDT is connected to an external circuit which has an internal pull up
, then SCDT will not stay low when no
video signal is present. The recommended circuit to keep SCDT low is shown Figure 9. For most applications,
Silicon Image recommends a pull down resistor of 1.5 KΩ. However, conditions within every design may vary.
Please use the calculations below to determine the proper pull-down resistor value.
SiI141B
SCDT(pin# 7)
external
chip
R
pull down resister
weak internal
pull down
internal pull up
Figure 9. Schematic for SCDT connected to external device with pull up
The external pull down resistor value depends on the pull-up circuit in the external device and can be calculated
with Equation [1] and [2] if the pull up is a passive circuit
. If the pull up is an active circuit, please consult the
manufacture of the other device.
The calculation for the maximum
resistor value is shown in the equation [1] below. In powered down mode, low
power consumption is achieved by making the resistor value as large as possible. Equation [1] determines the
maximum value of R while ensuring that SCDT stays lower than V
IL
of the external chip when SCDT goes into high
impedance. The small current flowing into the SiI141B internal pull down resistor is ignored in equation [1].
Equation [1a]
ILCCMAX
UpPull
VV
RR
R
<×
+
−
Equation [1b]
()
()
−
×
<
−
ILcc
ILUpPull
VV
VR
R
max
Example :
Pull-up resistor value is 10 KΩ, V
IL
of external chip is 0.8V, and maximum Vcc is 3.6V
R < 2,857ohms = (10 KΩ x 0.8V) / (3.6V – 0.8V)
The resistor value should be smaller than 2,857 KΩ.
The calculation for the minimum
resistor value is shown in the equation [2]. The minimum value is set so the
SCDT voltage exceeds V
IH
of the external chip in normal operation. In equation [2], the small current flowing into
the SiI141B internal pull-down resistor is ignored.