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In the function "I2C_rd()", there is a Wire.endTransmission() after the Wire.requestFrom(). That Wire.endTransmission() can be removed.
Explanation: Common-mistakes#2

Is the I2C_is_busy() for using the library in a interrupt ? I think that using the Wire library in a interrupt is already a bug on its own.

You use a repeated start between setting the register address and reading data. Every modern sensor should support that, but maybe there are somewhere I2C devices that don't like it.

A delay of 5 ms is a long time between the retries. I think that 1 ms is more than enough for every situation.

Are the retries an extra safety feature ? Do you keep track if that is really useful ? Maybe an Arduino as a Slave with a bad sketch can use it. When the I2C bus has noise or other troubles, those retries can not fix such I2C bus troubles.

When the Slave is an Arduino (with a bad sketch) a delay between writing the register address and requesting data is more helpful than retries.

Hello
Can i use this library for slave?
thank

hello,
why do you call 400 kHz Low Speed?
I2C_init(I2C_LOW); // init with low speed (400KHz)

the common definitions for i2c bus speed are the following:
Standard Mode (Sm) 100 kbit/s bidirektional
Fast Mode (Fm) 400 kbit/s bidirektional
Fast Mode Plus (Fm+) 1,0 Mbit/s bidirektional
High Speed Mode (Hs-mode) 3,4 Mbit/s bidirektional
Ultra Fast-mode (UFm) 5,0 Mbit/s unidirektional

so low speed should be used just for clock speed slower than standard,
400 kHz should be called I2C_FAST
and
100 kHz should be called I2C_STD

OTOH.
if there once sth like 9.6 kHz would be provided or needed, then you might call that I2C_LOW instead

Hello and thanks for providing this library. I had tried writing a wrapper for Wire with no luck :) I need to use is in the C header of another library (not main Arduino sketch).

I am no expert sorry if I ask a basic question:

I didn't understand how FRAM in examples is initialized. Is there any other definition that should be added for specific hardware? If I want to use it with an analog front end device, with slave address 0x5B, what would be the effect of writing:
I2C_SLAVE AFE ;

and then giving reference &AFE in initialization step?
Also, I see ci2c in most of the lines. Is it applicable to Samd21 cortex M0+ on MKR1000 boards?

Thanks again!