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DDC Signal Path
TMDS341A
SLLS702B – MAY 2006 – REVISED MARCH 2007
APPLICATION INFORMATION (continued)
Figure 38. Eye Diagram at Output 10-m 28-AWG Cable Figure 39. Eye Diagram Recovered by TMDS341A vs TP2
vs TP2 Eye Mask Eye Mask
Observed I
2
C bus voltage is dependent on bus resistance, capacitance, and time. The transient bus voltage,
when charging from a low state to a high state, can be calculated using equation (1).
V(t) = V
DD
(1 – e
–t/RC
) (1)
Where:
t is the time since the charging started
V
DD
is the pull-up termination voltage
R is the total resistance on the I
2
C link
C is the total capacitance on the I
2
C link
In the I
2
C bus specification, version 2.1, the high-level threshold voltage is V
IH
= 0.7 V
DD
, and the low-level
threshold voltage is V
IL
= 0.3 V
DD
.
From equation (1), the times to charge from a bus voltage of 0 V to the V
IH
and V
IL
levels are:
t
IH
= 1.204 × RC
t
IL
= 0.357 × RC
The bus rise time (from 0.3 V
DD
to 0.7 V
DD
) is then given by equation (2):
t
r(30-70)
= t
IH
– t
IL
= 0.847 × RC (2)
The TMDS341A can be easily applied in stand-alone switch boxes and digital displays. The following sections
show the bus lengths that can be supported in each case.
Maximum Bus Lengths for Switch Applications
Figure 40 shows the TMDS341A being used as a stand-alone switch. Both pull-up resistors are decided by the
source and sink equipment. A 1.5-k Ω resistor at the source and a 47-k Ω resistor at the sink are recommended.
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