I am new to using I2C so apologies if this has already been addressed
before.
In trying to communicate with a Honeywell HIH-6130 humidity sensor from
a RaspberryPi I ran into an apparent limitation of i2c-tools. There
doesn't seem to be support for reading multiple bytes of data from a
device that doesn't need a register selected before the read. This
device is accessed by first doing a write of just the address, followed
about 37ms later by a read of 4 bytes, as documented here
http://www.phanderson.com/arduino/I2CCommunications.pdf. The write
operation can be done by using write_quick(), and a subsequent
read_byte() will get the first byte of data, but multiple calls to
read_byte() will just get the same first byte again. Using
read_i2c_block_data() works better, however, because it sends an
unneeded command byte, I'm not sure the data returned is valid. There is
a long discussion of the details in this thread
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=32&t=29454.
It seems to me that what is needed is a way to optionally specify that
no command byte should be sent in read_i2c_block_data() and
write_i2c_block_data(). While this is not compliant with the SMBus
protocol, this device doesn't claim to be compliant, and the purpose of
these two functions seems to be to allow such communications. I don't
believe this particular device is unique so there should be other cases
where this is useful.
I looked at the python wrapper code in smbusmodule.c and it looks easy
enough to implement something like specifying a value of None for the
cmd argument to indicate that no command byte should be sent, but
implementation of the behavior seems to be buried somewhere deep in
i2c-dev.c or i2c-core.c and I haven't been able to find it. I see that
there is work being done in this project to clean up these drivers, so I
wanted to propose this change, and also offer my help.
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