I want to connect my esp32 with Arduino UNO that's why I have to run my Arduino at 3.3 v so while searching on google someone suggested replacing 16 MHz crystal with 8 MHz but I have 12 MHz crystal.
So can I use that 12 MHz to run Arduino at 3.3 v
if there is any other way please let me know
Thanks in advance
By connect, What do you mean?
do you mean connecting them with one power supply?
Or do you mean you want to connect each others input-output pins?
If its the first one, get a 3.3V voltage regulator
If its the latter, Get a level shifter.
replacing the crystal is no easy task and is basically way of destroying the arduino.
i am trying to do serial communication between Arduino and esp32 with RX and tx pins
avanishpratap:
i am trying to do serial communication between Arduino and esp32 with RX and tx pins
You need a level shifter.
Eventually the communication works without a level shifter - find out yourself. The 5V TX should be connected to the 3.3V RX with a voltage divider, the other direction may work with a direct connection.
It should be possible to set the prescaler to run the Arduino at 8MHz with 16MHz crystal/resonator.
The oscillator is not designed for 16MHz at 3.3V. For 8MHz the internal RC oscillator can be used, as AFAIR is done in the bootstrap.
Other stuff will get in the way then - like connecting the USB will provide 5volts . The internal regulator won’t be working .
The level shift is the better idea
Powering by USB is wrong, and flashing will not work properly when something else is connected to RX/TX. Disconnect the 3.3V device while connected to USB and everything is fine.
DrDiettrich:
Eventually the communication works without a level shifter - find out yourself. The 5V TX should be connected to the 3.3V RX with a voltage divider, the other direction may work with a direct connection.
Voltage divider -is- a level shifter. It's the most current wasting kind too. A diode and a pullup resistor make better for almost the same cost.
avanishpratap:
I want to connect my esp32 with Arduino UNO that's why I have to run my Arduino at 3.3 v so while searching on google someone suggested replacing 16 MHz crystal with 8 MHz but I have 12 MHz crystal.
So can I use that 12 MHz to run Arduino at 3.3 v
if there is any other way please let me knowThanks in advance
You can also make a stand-alone AVR chip "Duino" that runs at 3.3V (not super sure about stable 12MHz) and connects directly to to the ESP32.
Are you connecting via serial or SPI port?
If I have a 5V signal going to a 3V chip I can put a diode in that wire to stop 5V going to the 3V chip. On the 3V side I put a 4K to 10K resistor to charge the wire on that side of the diode when 5V is present and -not- getting through so that 3V is present and HIGH on the 5V side has 3V HIGH on that side. And when the 5V becomes 0V it sucks the 3V pullup through the diode and both sides see 0V.
Unlike the voltage divider this leveler only loses current when the 5V side is LOW instead of all the time. Serial logic is reversed, HIGH at idle and 0 signals, the diode & pullup serves it well.
3V TX works on a 5V serial RX, needs no leveling. With serial you only level 1 pin. With SPI you level 3 (MOSI, SCLK and Select) but it's faster.
avanishpratap:
i am trying to do serial communication between Arduino and esp32 with RX and tx pins
Generally a dumb idea.
Sounds like you are trying to pull a Lamborghini along with a donkey.
Can't you use only the ESP32.
Leo..
Wawa:
Generally a dumb idea.
Sounds like you are trying to pull a Lamborghini along with a donkey.
Can't you use only the ESP32.
+1 to that! 
And the UNO is always a poor choice. If you really wanted to interface at 3.3 V, you would just get a 3.3 V Pro Mini. But an ESP32 with port expanders if required, is the only sensible approach. 