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V(t) V

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TDP158

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SLLSEX2 – DECEMBER 2016

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9.2.2.2 DDC Pull Up Resistors

This section is for information only and subject to change depending upon system implementation. 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.

(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 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 28 summarizes the possible values of k under different threshold

combinations.

(2)

(3)

Table 28. Value k upon Different Input Threshold Voltages

Vth-\Vth+

0.7 VCC

0.65 VCC

0.6 VCC

0.55 VCC

0.5 VCC

0.45 VCC

0.4 VCC

0.35 VCC

0.3 VCC

0.1 VCC

1.0986

0.9445

0.8109

0.6931

0.5878

0.4925

0.4055

0.3254

0.2513

0.15 VCC

1.0415

0.8873

0.7538

0.6360

0.5306

0.4353

0.3483

0.2683

0.1942

0.2 VCC

0.9808

0.8267

0.6931

0.5754

0.4700

0.3747

0.2877

0.2076

0.1335

0.25 VCC

0.9163

0.7621

0.6286

0.5108

0.4055

0.3102

0.2231

0.1431

0.0690

0.3 VCC

0.8473

0.6931

0.5596

0.4418

0.3365

0.2412

0.1542

0.0741

From Equation 1, Rup(min) = 5.5 V/3 mA = 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.

If DDC working at standard mode of 100 Kbps, the maximum transition time T is fixed, 1 μs, and using the k

values from Table 28, the recommended maximum total resistance of the pull-up resistors on an I2C bus can be

calculated for different system setups. If DDC working in fast mode of 400 Kbps, the transition time should be set

at 300 ns according to I2C specification.

To support the maximum load capacitance specified in the HDMI spec, C(cable)(max) = 700 pF, C(source) = 50 pF,

CI = 50 pF, R(max) can be calculated as shown in Table 29.

Table 29. Pull-Up Resistor Upon Different Threshold Voltages and 800-pF Loads

Vth-\Vth+

0.7 VCC

0.65 VCC

0.6 VCC

0.55 VCC

0.5 VCC

0.45 VCC

0.4 VCC

0.35 VCC

0.3 VCC

UNIT

0.1 VCC

1.14

1.32

1.54

1.8

2.13

2.54

3.08

3.84

4.97

Ω

0.15 VCC

1.2

1.41

1.66

1.97

2.36

2.87

3.59

4.66

6.44

Ω

0.2 VCC

1.27

1.51

1.8

2.17

2.66

3.34

4.35

6.02

9.36

Ω

0.25 VCC

1.36

1.64

1.99

2.45

3.08

4.03

5.6

8.74

18.12

Ω

0.3 VCC

1.48

1.8

2.23

2.83

3.72

5.18

8.11

16.87

------

Ω

To accommodate the 3-mA drive current specification, a narrower threshold voltage range is required to support

a maximum 800-pF load capacitance for a standard-mode I2C bus.

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