HCLA Series
Miniature amplified low pressure sensors
I²C BUS
Introduction
The HCLA is capable to generate a digital output signal.
The device runs a cyclic program, which will store a corrected
pressure value with 12 bit resolution about every 250 µs
within the output registers of the internal ASIC. In order to
use the sensor for digital signal readout, it should be
connected to a bidirectional I²C-bus.
According to the I²C-bus specification, the bus is controlled
by a master device, which generates the clock signal,
controls the bus access and generates START and STOP
conditions. The HCLA is designed to work as a slave, hence
it will only respond to requests from a master device.
DATA valid (D):
State of data line represents valid data when,
after START condition, data line is stable for duration of
HIGH period of clock signal. Data on line must be
changed during LOW period of clock signal. There is
one clock pulse per bit of data.
Acknowledge (A):
Data is transferred in pieces of 8 bits
(1 byte) on serial bus, MSB first. After each byte receiving
device – whether master or slave – is obliged to pull
data line LOW as acknowledge for reception of data.
Master must generate an extra clock pulse for this pur-
pose. When acknowledge is missed, slave transmitter
becomes inactive. It is on master either to send last com-
mand again or to generate STOP condition in that case.
Slave address:
The I²C-bus master-slave concept requires
a unique address for each device. The HCLA has a
preconfigured slave address (1111000xb). By factory
programming it is possible to define a secondary slave
address additional to the general one. According to I²C
specification 127 different addresses are available. The
sensor will then listen to both slave addresses. After
generating a START condition the master sends the
address byte containing a 7 bit address followed by a
data direction bit (R/W). A "0" indicates a transmission
from master to slave (WRITE), a "1" indicates a data
request (READ).
DATA operation:
The sensor starts to send 2 data bytes
containing the current pressure value as a 15 bit
information placed in the output registers.
Digital I²C interface
The HCLA complies with the following protocol (Fig. 1):
Bus not busy:
During idle periods both data line (SDA) and
clock line (SCL) remain HIGH.
START condition (S):
HIGH to LOW transition of SDA line
while clock (SCL) is HIGH is interpreted as START con-
dition. START conditions are always generated by the
master. Each initial request for a pressure value has to
begin with a START condition.
STOP condition (P):
LOW to HIGH transition of SDA line while
clock (SCL) is HIGH determines STOP condition. STOP
conditions are always generated by the master. More than
one request for the current pressure value can be trans-
mitted without generation of intermediate STOP condition.
SCL
SDA
START
condition
Data
valid
Data allowed
to change
STOP
condition
1
S Slave Address R/W A
Data Byte 1
A
Data Byte 2
A
Data Byte 1
P
Read out of first pressure value
generated by master
generated by slave
S = START condition
A = Acknowledge
P = STOP condition
Read out of n pressure values (optional)
Data Byte 1 = High Byte (MSB first)
Data Byte 2 = Low Byte (LSB last)
Fig. 1:
I²C bus protocol
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