TMDS361 datasheet(25/42 Pages) TI

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RSCL

RSDA

9thClockPulse- AcknowledgeFromSlave

TSCL

TSDA

9thClockPulse- AcknowledgeFromSlave

OfSlave

OfDriver T

2C Pullup Resistors

TMDS361

www.ti.com............................................................................................................................................................................................ SLLS919 – DECEMBER 2008

Figure 31 illustrates the waveforms seen on the R-side I2C-bus when the master writes to the slave through the

I2C repeater circuit of the TMDS361. This looks like a normal I2C transmission, and the turnon and turnoff of the

acknowledge signals are slightly delayed.

Figure 31. Bus-R Waveform

Figure 32 illustrates the waveforms seen on the T-side I2C-bus under the same operation as in Figure 31. On the

T-side of the I2C repeater, the clock and data lines would have a positive offset from ground equal to the VOL of

the driver T. After the 8th clock pulse, the data line is pulled to the VOL 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 VOL 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

TMDS361 I2C repeater is below 0.4 V to be recognized by the device and then transmitted to the R-side I2C bus.

Figure 32. Bus T Waveform

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.

Rup(min) = VDD/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) = VDD(1 – e

–t/RC)

(3)

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