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ISSUE: Some sensors failed to init with "htu21df_init:htu21df did not reset" message.

FIX: Change line 124 of htu21dflib.c to
if ((buf[0] & 0xc7) != 0x02) {

Long description:
This issue caused by incorrect checking of user register content during sensor initialization routine.
As stated by manufacturer (see table on page 13 of sensor's datasheet https://cdn-shop.adafruit.com/datasheets/1899_HTU21D.pdf ), values of Reserved bits may be changed by manufacturer ("default
values of respective reserved bits may change over time without prior notice"). And I have real sensors with these bits changed (all '1').
So we need to mask these bits before checking.

PS: Excuse me for text description instead of attaching some diffs or patches. I am absolutely novice on Github.

Hi!
I hooked up a HTU21D-F Humidity Sensor (adafru.it/1899) to my raspberry pi A+. Was very lucky to find your code. I've installed it and received errors:
opening /dev/i2c-1
htu21df_init:htu21df I2C_RDWR failed -1/5
i2c_init failed -1

Now I'm stuck. Can you give me a hint?
"sudo i2cdetect -y 1" shows there must be something at dress 0x40.
So I think I've wired it right (VIN=Pin1 3.3VDC, GND=Pin9 Ground, SDA=Pin3 SDA1, SCL=Pin5 SLC1).
Stefan

In the htu21dflib.c file there is a line that is hard coded to the Raspbery V2 i2c value.

static const char I2CDEV[] = "/dev/i2c-1"; // raspberry pi

The version 1 Pi uses the i2c-0 address for the bus. You could maybe add a check for what version it is running on, and set the value accordingly.

Otherwise great code that works well!