Datasheet

RSCL
RSDA
9thClockPulse- AcknowledgeFromSlave
TSCL
TSDA
9thClockPulse- AcknowledgeFromSlave
V
OL
OfSlave
V
OL
OfDriver T
up(min) DD
R V / Isink=
t k RC= ´
( )
t/RC
DD
V(t) V 1 e
-
= -
TMDS261B
www.ti.com SLLS987A –SEPTEMBER 2009– REVISED JULY 2011
Figure 43 illustrates the waveforms seen on the R-side I
2
C-bus when the master writes to the slave through the
I
2
C repeater circuit of the TMDS261B. This looks like a normal I
2
C transmission, and the turnon and turnoff of the
acknowledge signals are slightly delayed.
Figure 43. Bus-R Waveform
Figure 44 illustrates the waveforms seen on the T-side I
2
C-bus under the same operation as in Figure 43. On the
T-side of the I
2
C 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 I
2
C repeater is below 0.4 V to be recognized by the device and then transmitted to the R-side I
2
C
bus.
Figure 44. Bus-T Waveform
I
2
C Pullup Resistors
The pullup resistor value is determined by two requirements:
1. The maximum sink current of the I
2
C buffer:
The maximum sink current is 3 mA or slightly higher for an I
2
C driver supporting standard-mode I
2
C
operation.
(1)
2. The maximum transition time on the bus:
The maximum transition time, T, of an I
2
C 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.
(2)
(3)
Table 4. Value of k for Different Input Threshold Voltages
V
th–
\V
th+
0.7 V
DD
0.65 V
DD
0.6 V
DD
0.55 V
DD
0.5 V
DD
0.45 V
DD
0.4 V
DD
0.35 V
DD
0.3 V
DD
0.1 V
DD
1.0986 0.9445 0.8109 0.6931 0.5878 0.4925 0.4055 0.3254 0.2513
0.15 V
DD
1.0415 0.8873 0.7538 0.6360 0.5306 0.4353 0.3483 0.2683 0.1942
0.2 V
DD
0.9808 0.8267 0.6931 0.5754 0.4700 0.3747 0.2877 0.2076 0.1335
Copyright © 2009–2011, Texas Instruments Incorporated 29