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2 I²C - communication
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• Valid data
Data is transmitted in bytes (8 bits) starting with the most significant bit (MSB). Each byte transmitted is
followed by an acknowledge bit. Transmitted bits are valid if, after a start condition, SDA remains at a
constant level during the high period of SCL. The SDA level may change only when the clock signal at
SCL is low.
• Acknowledge
An acknowledge after a transmitted byte is obligatory. The master must generate an acknowledge-relat-
ed clock pulse. The receiver (slave or master) pulls down the SDA line during the acknowledge clock
pulse. If no acknowledge is generated by the receiver, a transmitting slave will become inactive. A trans-
mitting master can abort the transmission by generating a stop condition and can repeat the command.
A receiving master may signal the end of the transfer to the transmitting slave by not generating an ac-
knowledge-related clock pulse at SCL. The digital pressure measuring cell changes to inactive interface
mode when processing internal command routines started by a previously sent command.
After every transferred byte (in both directions) the receiver of the byte gives feedback with the acknowl-
edge bit.The slave should always confirm the bytes by an ACK. If the slave does not respond with a LOW
level after the 8th bit, the master detects an exception (for example caused by requesting to the wrong
slave address). A NACK from the master’s side is not always an exception. It is also needed to terminate
a read data frame.
• Stop condition
Every data frame ends with a stop condition. A low-to-high transition on SDA while SCL is at the high
level indicates a stop condition. A command must be closed by a stop condition to start processing the
command routine in the digital pressure measuring cell.
2.5 Addressing
The first byte of every data frame contains the slave address and a R/W bit (Write = 0/Read = 1).
The default slave address (standard adress) of the digital pressure measuring cell transmitters is: 0x28,
another address can be set by using the software (
 see chapter 4.7.2 "Communication", Page 28). The
7 bit address of a I²C-device allows 112 bus addresses in general, because 16 of the 128 possible ad-
dresses are reserved for special purposes.
The digital pressure measuring cell has a fixed 5 bit-address, with a 2 bit-subadress, which is configu-
rable, but the device always answers to 0x28. So it is possible to use three digital pressure measuring
cells on one bus without getting collisions at the maximum. Possible addresses for the slaves in a bus
structure with one slave are 0x28, 0x29, 0x2A, 0x2B, in a bus structure with more than one slave 0x29,
0x2A, 0x2B. Other devices with other addresses (e.g. temperature sensors) can be added to the bus
too, as long as the maximum number of 112 devices is not exceeded.
Examples:
ADDR is 0x28 - data transfer from the master to the slave (write)
ADDR is 0x2B - data transfer from the slave to the master (read)
0 1 1 0 0 0 0 0
0x00 (write) 0x28 (address)
0x50 (first byte)
0 1 1 0 0 1 1 1
0x01 (read) 0x2B (address)
0x57 (first byte)