TMDS261B Datasheet, PDF (29/48 Pages) Texas Instruments – TWO-PORT HDMI SWITCH

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TMDS261B
SLLS987A – SEPTEMBER 2009 – REVISED JULY 2011
Figure 43 illustrates the waveforms seen on the R-side I²C-bus when the master writes to the slave through the
I²C repeater circuit of the TMDS261B. This looks like a normal I²C transmission, and the turnon and turnoff of the
acknowledge signals are slightly delayed.
9th Clock Pulse - Acknowledge From Slave
RSCL
RSDA
Figure 43. Bus-R Waveform
Figure 44 illustrates the waveforms seen on the T-side I²C-bus under the same operation as in Figure 43. On the
T-side of the I²C repeater, the clock and data lines would have a positive offset from ground equal to the V_OL of
the driver T. After the eighth clock pulse, the data line is pulled to the V_OL of the slave device, which is very close
to ground in this example. At the end of the acknowledge, the slave device releases and the bus level rises back
to the V_OL set by the driver until the R-side rises above VCC/2, after which it continues to be high. It is important
to note that any arbitration or clock-stretching events require that the low level on the T-side bus at the input of
the TMDS261B I²C repeater is below 0.4 V to be recognized by the device and then transmitted to the R-side I²C
bus.
9th Clock Pulse - Acknowledge From Slave
TSCL
TSDA
V Of Driver T
OL
V Of Slave
OL
Figure 44. Bus-T Waveform
I²C Pullup Resistors
The pullup resistor value is determined by two requirements:
1. The maximum sink current of the I²C buffer:
The maximum sink current is 3 mA or slightly higher for an I²C driver supporting standard-mode I²C
operation.
R_up(min) = V_DD / Isink
(1)
2. The maximum transition time on the bus:
The maximum transition time, T, of an I²C bus is set by an RC time constant, where R is the pullup resistor
value and C is the total load capacitance. The parameter, k, can be calculated from Equation 3 by solving for
t, the times at which certain voltage thresholds are reached. Different input threshold combinations introduce
different values of t. Table 4 summarizes the possible values of k under different threshold combinations.
t = k ´ RC
(2)
( ) ^{V(t) = V}DD ^{1- e-t/RC}
(3)
V_th–\V_th+
0.1 V_DD
0.15 V_DD
0.2 V_DD
0.7 V_DD
1.0986
1.0415
0.9808
Table 4. Value of k for Different Input Threshold Voltages
0.65 V_DD
0.9445
0.8873
0.8267
0.6 V_DD
0.8109
0.7538
0.6931
0.55 V_DD
0.6931
0.6360
0.5754
0.5 V_DD
0.5878
0.5306
0.4700
0.45 V_DD
0.4925
0.4353
0.3747
0.4 V_DD
0.4055
0.3483
0.2877
0.35 V_DD
0.3254
0.2683
0.2076
0.3 V_DD
0.2513
0.1942
0.1335
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