EM7180SFP

Copyright

2014, EM Microelectronic-Marin SA

7180SFP-DS Version 1.1, 10-Dec-14

13

www.emmicroelectronic.com

420005-A01, 2.0

I2C Sensor Interface (Sensor Bus)

SENtral coprocessor communicates with BMX055 sensor and serial EEPROM over I2C sensor bus (internal to SFP). Host MCU

can communicate directly with EEPROM and BMX055 sensor when the SENtral coprocessor is put into Pass-Through mode.

Note that Pass-Through mode is supported when SENtral is in Standby state (normal operation is suspended). Communication

speed in Pass-Through mode is limited to 400 kbit/s (Fast Mode).

I2C Pull-Up Resistance

The pull-up resistor value for both the host and sensor bus will depend on the I2C data rate and the number of devices on the

bus.

I2C mode

Rate

(kbit/s)

Rise Time

(ns)

Max Cb(pF)

4.7kΩ pull-up

2.47kΩ pull-up

Standard

100

1000

251.1

491.8

Fast

400

300

75.3

147.5

Fast Plus

High Speed- 1.7

MHz

1000

120

30.1

59.0

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

Table 12

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 re

quire a “Max Cb” value of >40 pF.

I2C mode

Rate

(kbit/s)

Rise Time

(ns)

Max Cb(pF)

4.7kΩ pull-up

2.47kΩ pull-up

Standard

100

1000

251.1

491.8

Fast

400

300

75.3

147.5

Fast Plus

High Speed- 1.7

MHz

1000

120

30.1

59.0

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

Table 12 I2C Pull-Up Resistance Table

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.