PNI SENtral - Manual (Page 16)

PNI Sensor Corporation Doc #1018049 R03
SENtral Technical Data Sheet Page 15
4.3 I
2
C Sensor Interface (Sensor Bus)
SENtral communicates with the accelerometer, gyroscope, and magnetometer over the sensor
bus, where SENtral acts as the I
2
C master and the sensors act as the I
2
C slaves. On the
sensor bus, SENtral initiates data transfer and generates the serial clock. SENtral’s I
2
C
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
2
C Pull-Up Resistance
The pull-up resistor value for both the host and sensor bus will depend on the I
2
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
2
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 80 20.1 39.3
Data 1700 160 40.2 78.7
High Speed-3.4 MHz
Clock 3400 40 10.0 19.7
Data 3400 80 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 .