ESP32 I²C 5V tolerance

[megabite]

Hello,

as part of my current ESP32 project, at some point I would like to link up my ESP32 WROOM with an Atmega328P-PU on a PCB, using I²C.

I've got a working perfboard prototype of the Atmega circuit, and on it, I've pulled the I²C SDA and SCL pins up to 5V with two 4K7 resistors. This seemed like a good idea because the Atmega and ESP32 will be installed inside a car with about 15 ft of wire between them on the I²C bus, and the Atmega itself runs on 5V in the first place.

Now, I think I read somewhere that the ESP32 I²C pins are 5-volt tolerant, provided that you use higher-value resistors, maybe like my 4K7 ones.

Is this true, or do I need a level shifter of some kind to connect the Atmega perfboard circuit and the ESP32?

[WiFive]

A level shifter at esp32 side would be a good idea, also some transient protection. And you may want to consider using pjon.

[megabite]

Would a level shifter simply be "a good idea", or will it actually be needed? Can the I²C pins on the ESP32 tolerate 5V or not?

Transient protection is taken care of, I will be using a TVS diode combined with a fuse and a choke, in front of a 3V3 DC converter down from the car's 12V to power the ESP32. And the Atmega PCB will have a similar setup, except with a 5V voltage converter.

On the other hand, the Atmega can be run at 3V3. Could I just change the Atmega's power supply from 5V to 3V3 and then run the I²C bus across those 15 ft of wire on 3V3? Or would that just not be enough to ensure a reliable I²C data transfer?

[ESP_Sprite]

Not sure if you can just install current limiting resistors in this situation... I2C is both open-drain and bidirectional, so at least in one direction, the current limiting resistors will form a resistive divider with the pull-up resistors I2C requires. I'd go for a real level shifter with this if you want to use this at 5V. Another option, however, may be to run the I2C lines at 3.3V (as in: connect the required pull-ups to 3.3V instead of 5V): if I recall correctly, the ATMega at 5V will happily recognize these signals as valid and you won't need any shifters. Because I2C is open-drain, even when the ATMega is run at 5V, it will only pull the I2C lines down and never up to 5V.

[megabite]

I've just ordered a two-channel bi-directional level shifter off eBay for this specific purpose.

Considering I've got 15 ft of wire to overcome in a noisy car environment, it's probably really best to run the I²C bus at 5V and then only shift it down to 3V3 at the ESP32 input pins.

[WiFive]

I meant transient protection on the gpio lines. I still say you should consider differential signaling or pjon instead of i2c.

[Gfast2]

According to ATmega328p Datasheet, you can even run you Atmel MCU as low as @1.8V.

If you do Arduino. As far as I can remember, there is also some Arduino Micro Pro running @ 3.3V. Perhaps you wanna check them out.

Cheers

Gfast2

[megabite]

I think I will still keep the Atmega running at 5V. I'm too far into the project already with the Atmega to change it to 3V3, as a bunch of sensor readings depend on the 5V for reference. I would have to make fundamental changes to my code and my perfboard layout.

Just keeping it on 5V as it is seems the better idea, and then just put a $1 level shifter between the Atmega and the ESP32. And my guess is that along 15 ft of wire inside a car between the Atmega and the ESP32, 5V guarantees a more stable I²C than 3V3. I've had people telling me that I²C over such a distance is a very bad idea in the first place, but all I can say is that at home in my workshop, it worked perfectly if I pulled the wires up to 5V with 4K7 resistors, as is the general advice when using the I²C bus on the Arduino.

[Gfast2]

Many years before, I did a project using rs-485 chips (sn75176). It's dirty cheap and helps me programming my Arduino (Mega 2560) from a distence about 15m-20m away. If the I²C in your project not rocks, try this one.

Cheers

Gfast2

[ESP_Angus]

Hi megabite,

To add a little bit to the existing range of comments here:

• To meet the ESP32 "maximum voltage" specification, you can't pull ESP32 pins up to 5V. So if this is, for example, going in a product you will sell or rely on for some very important function, don't do that.
• However, pulling up to 5V via 4.7K resistors will only bleed approx (5-3.3)/4700 amps of current into the ESP32's snapback overvoltage protection circuit. So approx 0.3mA. This is probably not enough to cause any damage or malfunction. So if this is a personal project, you can probably take the risk.
• If you don't want to take the risk, you can use any two cheap N-channel FETs to level shift I2C (NXP has an app note for this: http://www.nxp.com/documents/applicatio ... N10441.pdf )

However, as has also come up, I'd be a lot more worried about the 15 feet of single-ended (like I2C) wiring inside a noisy electrical environment like a car and no protections. Transients induced in the cable are probably more likely to damage something than the 0.3mA of I2C pullup current! (Or, even more likely, induce enough noise for bad I2C transactions.)

• I worked on a product a few years ago where we added an I2C differential transceiver - PCA9614DP - to deal with this kind of electrically noisy environment. Or as GFast2 says you could try an inherently differential protocol like RS-485.
• If you want to keep it simple with single-ended signals (ie I2C), consider adding a low value series resistor and a protection diode at both ends of both I2C lines. Something like R3,R4,D1 & D2 in the first image here: https://electronics.stackexchange.com/q ... on-circuit (choose R3/R4 values in the 50-100 ohm range and for D1/D2 a TVS or failing that a zener diode with a breakdown voltage above 5.5V.) You'll still only need the pullups (R1/R2 in that picture) at one end, unless you're level shifting.
• Consider how you're doing grounding, as the two ends will need to see the same ground for single-ended signalling to work. Traditional automotive grounding via the chassis will also be noisy (there's a reason all automative data buses are differential!)
• May work fine without these in your car, who knows?!? Let us know what you end up doing and how it works out.