Vaisala HUMICAP HMM105, Technical Reference

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Chapter 2 _______________________________________________________________ I2C Interface
VAISALA _____________________________________________________________________ 5
Communication Flow
Basic communication flow always includes I
2
C write and read
commands. First the master writes a command to the slave device, and
then the master reads the results of that command from the slave.
When the master reads data from the slave, there is an ACK/NACK-bit in
the status byte that informs the master whether or not communication
was success. ACK is defined as zero and NACK is defined as one in the
logical level.
NOTE This ACK/NACK refers to message level acknowledge. I 2 C protocol
includes separate byte level acknowledge.
Command messages sent by the master are called
invokes
. Replies sent
by the slave are called responses . Note that also responses include the
device address.
Table 6 Invoke Message in HMM105 I 2 C Interface
Message Segment Length Content
Start I
2
C start condition.
I
2
C address 1 byte See Table 4 on page 4.
Command 1 byte Identifier of command.
Device address 1 byte See Table 5 on page 4.
Frame length 1 byte Length of invoke message in bytes
(excluding I
2
C address, including CRC).
Minimum invoke frame length is 5 bytes.
Data Variable length Content and length depending on the
command. May be left out if command
contains no data in the invoke message.
Checksum 2 bytes See section Checksum on page 9.
Stop I
2
C stop condition
Table 7 Response Message in HMM105 I 2 C Interface
Message Segment Length Content
Start I 2 C start condition.
I 2 C address 1 byte See Table 4 on page 4.
Status 1 byte See section Status Byte on page 9.
Command 1 byte Identifier of the command that was used
in the invoke that the slave is
responding to. If slave is in idle state (no
valid invoke), command code is FFh.
Device address 1 byte See Table 5 on page 4.
Frame length 1 byte Length of the response message in
bytes (excluding I 2 C address, including
CRC). The minimum response frame
length is 6 bytes.
Data Variable length Content and length depending on the
command.
Checksum 2 bytes See section Checksum on page 9.
Stop I
2
C stop condition