PCF85063TP V_IH maximum value for I2C operation

[benthacher]

Hello, I'm designing a circuit with the PCF85063TP RTC powered from a Li-Ion cell (3V-4.2V) that I want to always be powered, interfacing with a microcontroller operating at 3.3V. If the cell is fully charged, I don't see any issue, as the threshold for minimum high-level input voltage for the I2C lines (pulled up to the 3.3V supply) would be 4.2 * 0.7 = 2.94, which 3.3V exceeds, but when the cell is at or below 3.3V, I don't know what will happen. If VDD of the RTC is 3.0V for instance, is the high value of 3.3V on the I2C lines going to be an issue? The datasheet says the maximum value for the high level input voltage is VDD, so if I give it 0.3V over that value, will the chip break or not register the I2C data? Thanks!

[benthacher]

Hi David,

Thanks for the quick reply! It looks like the output voltage only applies to the CLKOUT pin as that's the only output on the device that's push-pull, so it'll connect the pin internally to VDD. The I2C pins SDA and SCL are open drain I/O pins, so it relies on the I2C lines being pulled up to VDD (in my case I have them pulled up to 3.3V). So the question still remains, if the battery is at 3V, would the I2C lines being pulled up to 3.3V (greater than the RTC's supply as it's connected to the battery directly) be an issue for the chip? The datasheet does say the maximum input voltage is VDD (in this scenario 3V) but I'm hoping it doesn't matter that much. Another sensor in this design doesn't specify a maximum input voltage for SDA/SCL but that seems more like an oversight and less like a feature. Not sure if there's any deeper information to find other than the values in the datasheet though. I thought initially that I could pull up the I2C lines to the battery voltage as that should satisfy most situations but the MCU can take a maximum of 4V on input pins, so that won't work.

[diazmarin09]

Hello benthacher,

Thank you for contacting us. It is our pleasure.

Please note that we recommend supplying both, the MCU and RTC devices, with the same supply voltage.

“So the question still remains, if the battery is at 3V, would the I2C lines being pulled up to 3.3V (greater than the RTC's supply as it's connected to the battery directly) be an issue for the chip?”

In such a case, the circuit might work but is not guaranteed. You may take care of the parameters mentioned on the table provided before (VIH, VIL, VOH and VOL). In my personal opinion, I don't believe it will cause any damage to the device.

I hope this information helps.

Regards,

David

[diazmarin09]

Hello benthacher,

I hope all is great with you. Thank you for using the NXP communities.

Correct. If the MCU operates at 3.3V, there may be an issue if the battery voltage drops.

You may consider the following parameters for your calculations:

Let's say that your battery voltage is 3V, it means that the PCF85063TP device will output:

VOH: about 2.4V

VOL: up to 0.6V

With such values, the communication is limited as the MCU expects VIL: up to 1V and VIH to about 2.31.

Therefore, the circuit might work, but I do see a potential issue if the battery voltage decreases to 3V or lower. It is recommended to supply both the MCU and the RTC with the same voltage level.

I hope this information helps.

Regards,

David