Re: [lm-sensors] Support for non-register devices

[Guenter Roeck <linux-0h96xk9xTtrk1uMJSBkQmQ@public.gmane.org>]

On Wed, Mar 06, 2013 at 02:02:41PM -0800, Joe Buehl wrote:
> 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.
> 
Redirecting to i2c-dev mailing list.

Guenter