i2c
I'd like to interface an MPU-9150 sensor with an ATmega128 using I2C. The Atmega operates at 5V while the sensor chip operates at 3.3V.
How can I find out if the two can work together? I'm not very used to reading data sheets and don't know what to look for.
A quick parametric search on Digikey shows that the ATTiny20 is the cheapest AVR with dedicated I2C capabilities in one-off quantities. Note that the ATTiny20 only provides hardware slave support, master TWI/I2C still needs a software implementation.
A slight step up is the ATTiny25 which has both master and slave TWI/I2C support, but operating in master mode the clock generation I believe has to be generated in software.
For complete TWI/I2C support, take a look at the ATTiny48.
I think I got it.
Many I2C devices connected to 5V and 3.3V I2C line (pullups connected to 3.3V) will not work.
High state for many devices is 0.7Vdd.
ADT7420 temperature will not work too - it has minimum high state level = 0.7Vdd.
For 5V Vdd minimum high level is 3.5V (above 3.3V). With 3.3V pullups 5V powered device will read line always as low state.
DS1307 is working, because minimum high state input level is 2.2V (below 3.3V).
Best Answer
From the datasheet for the ATmega128: ATmega128 datasheet
The input High Voltage can be a minimum of 0.6 of Vcc. If you have your ATmega128 running at 5V, the minimum HIGH input is 3V.
If you want to be safe, try googling "3.3V to 5V level shifter" and you'll get lots of resources for creating circuits to use.