CFA533 i2C logic levels

[johnny.dr]

The CFA533 is specified as a CMOS part. Are the logic levels TTL compatible? I was planning to power the module by 5V and communicate using i2C with pull-ups connected to a 3.3V rail. Is this possible? Thanks!

Crystalfontz LCD Blog

Looking for additional LCD resources? Check out our LCD blog for the latest developments in LCD technology.

 

[CF Tech]

"TTL" is not correct. It is CMOS logic.

Why power the module from 5v? It will work fine at 3.3v. Well, if your 3.3v is derived from your 5v, that might make sense, power wise.

I think you would want to make your pull-ups go to 5v, maybe with a current limiting resistor to make sure not too much current is driven into your processors pins by the pull-ups.

I am worried that pulling up to 3.3v might not reliably trigger a "1" on the CFA533 side.

 

[johnny.dr]

Why power the module from 5v? It will work fine at 3.3v. Well, if your 3.3v is derived from your 5v, that might make sense, power wise.

Exactly. That and my i2C pins are not 5V tolerant. I'll just plan on using a level translator. Are the 533's i2C pins true open drain connections or totem-pull outputs?

 

[johnny.dr]

On second thought I think I'll just power it with 3.3 and be done =/ thanks!

 

[CF Tech]

. . . Are the 533's i2C pins true open drain connections or totem-pull outputs?

Yes, for I2C those pins are set to open drain, but they still have the protection diode that would cut in if the pin were pulled above the module's power supply.