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 -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
