SENSIRION Reading sensor information Application Note

Application Note for I2C Flow and Differential Pressure Sensors

Reading scale factor, measurement unit and tracking information

Summary

Sensirion provides I2C sensors measuring flow or
differential pressure. These sensors are based on the
same thermal flow sensor chip, called SF04. The chip
contains an EEPROM memory cell, where amongst others

1. Introduction

certain sensor information as scale factor, measurement
unit and tracking information is stored. The present
application note explains how to read out this sensor
information.

The output of sensors with a SF04 c hip inside is a 16bit
integer number provided by an I
physical value of the measurement, one ha s to divide the
output by a scale factor and the measurement unit has to
be known. Scale factor and measurement unit are defined
during calibration and cann ot be changed later on. During
calibration, the information about the used sc ale factor an d
measurement unit is written to the EEPROM, which can

2

C interface. To get the

2. Read EEPROM

For basic sensor communication, please refer to the
product datasheet or the user manual “Sensirion I2C Flow
and Differential Pressure Sensors”.

To dump the content of the EEPROM starting at a giv en
EEPROM address, two steps have to be performed.

1st Step

The internal address register ha s to be set by invoking a
set EEPROM address command.

8 Bit Command Code Command

hFA set EEPROM address
Important: The EEPROM address is a 12-bit value

followed by additional 4 bits which are not interpreted by

be read out by the user. Furthermore the serial numbers of
the flow chip and the sensor as well as sensor name and
item number are stored in the EEPROM. With this se nsor
information, the production process is fully traceable.

The following chapters describe how to read out sensor
information as scale factor, measurement unit and serial
number.

the sensor (can be sent as 0). Example: To set the
EEPROM address h2B6, the full command string is: 80 FA

2B 60. (hexadecimal, assuming the default I2C address)

2nd Step

Send an I2C header with R/_W=1. After the header with
R/_W=1, the sensor system writes the EEPROM word at
the actual address to the bus, followed by a CRC byte.
The internal address register is automatically increased
after each completed word write out. Therefore, if the
master continues clocking the SCK line after the CRC byte,
the next EEPROM word with corresponding CRC byte is
written to the bus, and so on. The master can interrupt
EEPROM dumping by not pulling down the DATA line
during any acknowledge-related SCK p ulse (NACK).

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Application Note: Reading sensor information

1

1

I2C Read EEPROM

123456789 19 2221 2720 23 24 25 26

I2C header (R/_W=0)

10 11 12 13 14 15 16 17

W

Cmd (hFA)

ACK

123456789

S RI2CAdr

I2C header (R/_W=1)

10 1312 19 2221 2711 14 15 16 17 20 23 24 25 2618 28 3130 3629 32 33 34 35

18

ACK

LSByte EEData

ACK

LSByte EEData

ACK

ACK

3. Scaling of Measurement Signal and Unit

To get the physical information from the sensor reading,
the scale factor and the measurement unit have to be
known. For some standard sensors, this information can
be found in the datasheet. For all other sensor s it has to
be read from the Sensor internal EEPROM memory.

A series of operations is needed for correct scaling and
determination of the measurement signal.

The original calibrated signal read from the sensor is an
integer number. The integer value can be converted to the
original unit the sensor has been calibrated for. For this
the number has to be divided by the sensors ‘scale factor’
which is stored in the EEPROM.

Again, the EEPROM address is composed of an EEPRO M
base address and an address offset. To determine the
base EEPROM address for actual sensor inform ation, bit
<6:4> of User Register (called active configuration field)
must be multiplied by h300. Then the Wordadr Offset of
following table must be added to that address.

28 3130 3629 32 33 34 35

EEAdr XXX XEEAdrS I2CAdr

ACK

ACK

ACK

Check ByteMSByte EEData

Check ByteMSByte EEData

ACK

ACK

P

NACK

(Example: address of article code

- bit<6:4> of User Register = '010'

-> EEPROM base address = 2 * h300 = h600

-> EEPROM address = h600 + h2E8 = h8E8

-> article code can be found at addresses h8E8 to h8F7)

Wordadr

#Words Description/Format

Offset

h2B6
h2B7

scale factor
measurement unit (for coding

see below)

Note: The scale factor and the measurement unit were
written to the EEPROM during calibration as information
about the used calibration parameters. Scale factor and
measurement unit cannot be changed after calibration.

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Application Note: Reading sensor information

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10

6

2

4. Measurement Unit Coding

The 16bit flow unit code includes different types of
information:

1. dimensions (e.g. milli, 0.001) (16 possibilities)

2. time base (e.g. per second) (16 possibilities)

3. unit (e.g. standard liter) (32 possibilities)

4. reserved (8 possibilities)

Bit <3:0> (x*1) Dimension Prefix

0 – 2 t.b.d.

3 1e-9 n
4 1e-6 u
5 0.001 m
6 0.01 c
7 0.1 d
8 1 1

9 10 ­10 100 h
11 1000 k
12 1e6 M
13 1e9 G

14 – 15 t.b.d.

Bit <7:4>

(x*16)

Time Base Comment

0 no time base e.g. pressure /

totalized flow
1 per microsecond us
2 per millisecond ms
3 per second s
4 per minute min

5 per hour h
6 per day day

7 – 15 t.b.d.

Bit <12:8>

(x*256)

10 – 15 t.b.d.

20 – 31 t.b.d.

Bit <15:13> (x*8192) are not defined

Examples:

Unit Code

nl/s 8*256 + 3*16 + 3 = 2099
m3/s 8*256 + 3*16 + 11 = 2107

mln/min 0*256 + 4*16 + 5 = 69

hPa 16*256 + 0*16 + 10 = 4106

Volume / Pressure Comment

0 norm liter

(0°C, 1013 hPa)

1 standard Liter

(20°C, 1013 hPa)

2 – 7 t.b.d.

8 liter (liquid) l, typ. liquid flow
9 gram g, typ. liquid flow

16 pascal Pa, pressure
17 bar bar, pressure
18 meter H2O m H2O, pressure
19 inch H2O in H2O, pressure

nl, typically for gas
flow

sl, typ. gas flow

5. Tracking information

Some tracking information can be read out from the
EEPROM, as the serial number, part name and item
number of the sensor.

The EEPROM address is composed of an EEPROM base
address and an address offset. To determine the base
EEPROM address for general sensor information, bit
<2:0> of Read-Only Register 2 (called active config uration
field at boot time) must be multiplied by h300. Than the
Wordadr Offset of the following table must be added to
that address.

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(Example: address of article code

- bit<2:0> of Read-Only Register 2 = '010'

-> EEPROM base address = 2 * h300 = h600

-> EEPROM address = h600 + h2E8 = h8E8

-> article code can be found at addresses h8E8 to h8F7)

Wordadr

Offset

h2E4-h2E7
h2E8-h2F1

h2F2-h2F7
h2F8-h2F9

#Words
(16 bit)

Description/Format

serial number chip (h2E4: MSB,
h2E7: LSB)
Part name
Item number
serial number product

Application Note: Reading sensor information

6. Revision history

Date Version Changes

June 2009 V1.0 Initial Release

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