user19878: I am trying to figure if SDA and SCL pins from Arduino Uno is a TTL or a CMOS technology. I am not able to find this info anywhere but regarding it is a I2C protocol it does make sense to be a TTL technology. Can someone kindly confirm this info, please?

user3879: Your question makes no sense. I²C uses open-drain/open-collector drivers which can even be discrete transistors or FETs although the inputs correspond to CMOS/TTL. These can be mixed - the Arduino uses MOS technology.
Of more significance is the supply voltage and the high/low trigger points (and of course the value of the mandatory pullup resistors which determine speed and/or range).
It is possible to use mixed 3.3V/5V levels. I use a Pi (3.3V) to communicate to Arduino (5V) which works (provided pullup is to 3.3V) although technically the Arduino input high levels are marginal.

user4143: It is CMOS in that the IO pins of the MCU use MOSFETs not BJTs.
However I2C is slightly different in that it is open-drain not push-pull, so it's not really either TTL or CMOS since it doesn't conform to either logic level standard (since there is no "high" voltage, only what is pulled up by the external resistor). That said, since the ATMega is normally CMOS when in input or push-pull mode the logic levels of the I2C should conform to CMOS not TTL in order for the Arduino to read them properly.
It's closer to RTL than TTL or CMOS.

19878: <p>I am trying to figure if SDA and SCL pins from Arduino Uno is a TTL or a CMOS technology. I am not able to find this info anywhere but regarding it is a I2C protocol it does make sense to be a TTL technology. Can someone kindly confirm this info, please?</p>
 3879: <p>Your question makes no sense. I²C uses open-drain/open-collector drivers which can even be discrete transistors or FETs although the inputs correspond to CMOS/TTL. These can be mixed - the Arduino uses MOS technology.</p>

<p>Of more significance is the supply voltage and the high/low trigger points (and of course the value of the mandatory pullup resistors which determine speed and/or range).</p>

<p>It is possible to use mixed 3.3V/5V levels. I use a Pi (3.3V) to communicate to Arduino (5V) which works (provided pullup is to 3.3V) although technically the Arduino input high levels are marginal.</p>
 4143: <p>It is CMOS in that the IO pins of the MCU use MOSFETs not BJTs.</p>

<p>However I2C is slightly different in that it is open-drain not push-pull, so it's not really either TTL or CMOS since it doesn't conform to either logic level standard (since there is no "high" voltage, only what is pulled up by the external resistor). That said, since the ATMega is normally CMOS when in input or push-pull mode the logic levels of the I2C should conform to CMOS not TTL in order for the Arduino to read them properly.</p>

<p>It's closer to RTL than TTL or CMOS.</p>
