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SENtral Technical Data Sheet Page 15

4.3 I

C Sensor Interface (Sensor Bus)

SENtral communicates with the accelerometer, gyroscope, and magnetometer over the sensor

bus, where SENtral acts as the I

C master and the sensors act as the I

C slaves. On the

sensor bus, SENtral initiates data transfer and generates the serial clock. SENtral’s I

sensor interface supports Standard mode with a rate up to 100 kbit/s, Fast mode with a rate

up to 400 kbit/s, and Fast Plus mode with a rate up to 1000 kbit/s.

The two wires comprising the sensor bus are SDAM, the serial data line, and SCLM, the

serial clock. Both are bidirectional and driven by open drain transistors within SENtral.

Each line should be attached to a pull-up resistor, which is further discussed in Section 4.4.

4.4 I

C Pull-Up Resistance

The pull-up resistor value for both the host and sensor bus will depend on the I

C data rate

and the number of devices on the bus. Table 4-2 provides the maximum acceptable bus

capacitance, as a function of bus rate, which can be accommodated with a 4.7 kΩ or 2.4 kΩ

pull-up resistor. As a general rule, each device connected to the bus represents 10 pF of

capacitance on the bus, so a bus with 4 devices would require a “Max Cb” value of >40 pF.

Table 4-2: I

C Pull-Up Resistance Table

I2C Mode

Rate

(kbit/s)

Rise Time

(ns)

Max Cb (pF)

4.7 kΩ pull-up

2.4 kΩ pull-up

Standard

100

1000

251.1

491.8

Fast

400

300

75.3

147.5

Fast Plus

1000

120

30.1

59.0

High Speed-1.7 MHz

Clock

1700

20.1

39.3

Data

1700

160

40.2

78.7

High Speed-3.4 MHz

Clock

3400

10.0

19.7

Data

3400

20.1

39.3

As the table implies, for most Standard and Fast Mode implementations a 4.7 kΩ pull-up

should work well, while a 2.4 kΩ pull-up normally should be used for Fast Plus. See Section

7.1 of NXP’s UM10204 specification for additional information.

http://www.nxp.com/documents/user_manual/UM10204.pdf.

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