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TSDA

TSCL

TMDS141

RSDA

RSCL

TSDA

TSCL

TMDS141

RSDA

RSCL

SDA

SCL

BUS

MASTER

SDA

SCL

BUS

SLAVE

TSDA

TSCL

TMDS141

RSDA

RSCL

3.3V

Source

Sink

Repeater

Rup

SOURCE

Rup

SINK

Rup

SINK

Rup

SOURCE

I2C Pull-up Resistors

up(min) + VDD lsink

(1)

T + k

(2)

V(t)

+ V

* e*t RC)

(3)

TMDS141

SLLS737C – JUNE 2006 – REVISED JUNE 2007

APPLICATION INFORMATION (continued)

Figure 23. Typical Series Application

The pull-up 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.

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 pull-up

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 1 summarizes the possible values of k under different

threshold combinations.

Table 1. Value k Upon Different Input Threshold Voltages

Vth-\Vth+

0.7VDD

0.65VDD

0.6VDD

0.55VDD

0.5VDD

0.45VDD

0.4VDD

0.35VDD

0.3VDD

0.1VDD

1.0986

0.9445

0.8109

0.6931

0.5878

0.4925

0.4055

0.3254

0.2513

0.15VDD

1.0415

0.8873

0.7538

0.6360

0.5306

0.4353

0.3483

0.2683

0.1942

0.2VDD

0.9808

0.8267

0.6931

0.5754

0.4700

0.3747

0.2877

0.2076

0.1335

0.25VDD

0.9163

0.7621

0.6286

0.5108

0.4055

0.3102

0.2231

0.1431

0.0690

0.3VDD

0.8473

0.6931

0.5596

0.4418

0.3365

0.2412

0.1542

0.0741

From equation 1, Rup(min) = 5.5V/3mA = 1.83 kΩ to operate the bus under a 5-V pull-up voltage and provide less

than 3 mA when the I2C device is driving the bus to a low state. If a higher sink current, for example 4 mA, is

allowed, Rup(min) can be as low as 1.375 kΩ.

Given a 5-V I2C device with input low and high threshold voltages at 0.3 Vdd and 0.7 Vdd, the valued of k is

0.8473 from Table 1. Taking into account the 1.83-k

Ω pull-up resistor, the maximum total load capacitance is

C(total-5V) = 645 pF. Ccable(max) should be restricted to be less than 545 pF if Csource and Ci can be as heavy as 50

pF. Here the Ci is treated as Csink, the load capacitance of a sink device.

Fixing the maximum transition time from Table 1, T = 1

µs, and using the k values from Table 1, the

recommended maximum total resistance of the pull-up resistors on an I2C bus can be calculated for different

system setups.

To support the maximum load capacitance specified in the HDMI spec, Ccable(max) = 700pF/Csource = 50pF/Ci =

50pF, R(max) can be calculated as shown in Table 2.

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