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SDAn
Switch Box
TMDS341A
SDA_Sink
V
DDsink
R
upsink
C
sink
C
cable2
C
O
C
I
C
cable1
C
source
V
DDsource
R
upsource
Source
Sink
C
source
Sink
TMDS341A
SDA_SinkSDAn
Source
C
cable
C
sink
DVI/HDMI RX
V
DDsource
R
upsource
V
DDsink
R
upsink
TMDS341A
SLLS702B – MAY 2006 – REVISED MARCH 2007
APPLICATION INFORMATION (continued)
Figure 40. DDC Link from Source to Sink With External Switch Box
R
upsource
= 1.5-k Ω pull-up to 5 V
R
upsink
= 47-k Ω pull-up to 5 V
R
total
= R
upsource
// R
upsink
= 1.45 k Ω
C
total
= C
source
// C
cable1
// C
i
// C
o
// C
cable2
// C
sink
For standard mode I
2
C, the frequency is at 100 kHz, and the transition time must be less than 1 µs. The total
allowable capacitance, C
total
, is then 814-pF. C
source
and C
sink
are limited by the HDMI specification to 50 pF.
C
i/o
for the TMDS341A is 10 pF max. The total capacitance from DVI or HDMI cables, C
cable1
and C
cable2
,
should then be less than 704 pF.
Typical capacitance is 200 pF for a 28-AWG 5-m HDMI cable and 300 pF for a 28-AWG 5-m DVI cable. The
recommended total cable length is the length of cable 1, Lcable1, plus the length of cable 2, Lcable2. For a
28-AWG DVI cable, the total cable length is 11 m; and for a 28-AWG HDMI cable, the total cable length is 17
m.
This calculation is applicable to V
IH
≤ V
pass
.
Maximum Bus Lengths for DTV Applications
Figure 41 shows the TMDS341A being used as a switch in a DTV and being placed on the same PCB board as
the DVI/HDMI receiver. Unlike Figure 40 , the output connector of the TMDS341A stand-alone switch and the
input connector of the sink are removed, which results in a lower capacitance in the DDC link and eliminates the
impedance discontinuity. However, the capacitance of the removed connectors is relatively small, relative to the
total allowable capacitance. The results from the previous section Maximum Bus Lengths for Switch Applications
can be reused if the pull-up resistors and capacitances have the same values. The recommended total cable
length is the length from source to sink.
Figure 41. DDC Link From Source to Sink Without External Switch Box
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