While reading about I2C/TWI protocol, i recognized as important part the ability to pull the signal down by any member of the communication. Thus pull-up resistors being very important here.
There are few schematics I found: http://www.cypress.com/documentation/application-notes/an50987-getting-started-i2c-psoc-1 https://cs.wikipedia.org/wiki/I%C2%B2C https://www.superhouse.tv/i2c-for-arduino/ All of these schematics contains VCC source for SDA and SCL.
What I do not get is where/what is voltage source in the case of ATMEGA & sensor (or two atmels cpus). In all schematics I have found there are only two cables - SDA ~ SDA and SCL ~ SCL.
I cannot grasp how would that work if there is not external VCC source. Sorry if the answer lies somewhere here on the stack, I could not find it.
As requested - links with vcc missing
In here https://howtomechatronics.com/tutorials/arduino/how-i2c-communication-works-and-how-to-use-it-with-arduino/ - there is actually both of the schematics (in the beggining one with the external voltage connected through resistor to SDA & SCL) but in the schematics with arduino and two moduls, no vcc.