Can anyone explain me why Compass Module, which uses I2C requires I socket, while Temp&Humidity Module, which also uses I2C, requires X, Y?
What are the advantages of using dedicated I2C lines over GPIO ports as I2C?
Why Temp&Humidity Module cannot use O and S sockets (their 3rd and 4th pins are GPIO)?
SHT1x is NOT an I2C device, no matter what the GHI documentation says 
It’s more like an SPI device, using a two-wire serial protocol (data and clock). The driver I suspect uses bitbang IO (basically software SPI with reduced pin count, no CS line)
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You are absolutely right - it is not I2C.
And I think I have figured out the answer to my second question: every S and O socket will be marked with X.
@ iamin -
- This is simply a software-driven version of the protocol
- A dedicated bus is much faster than software, however I and S sockets are limited compared to GPIO sockets. The general reason for running a module via software is just to free up sockets that could be used for more important things.
- Actually, it is due to the fact that the data and clock pins are exposed on pins 3 & 4 while the I and S sockets require the data and clock pins to consume pins 7-9 for SPI and 8 & 9 for I2C