Problems with powering Pmod Grove RTC using backup battery

[Mahdi]

Hello,

I am using a Pmod Grove RTC with a PYNQ board, and it seems to be operating normally when it is powered directly from the 3.3 V of the board. However, when I plug in the backup battery (3.0 V CR1225) into the grove RTC and have it also connected to the board, I can not read or write to the clock. It seems the backup battery somehow disables the device. After reading the page 3 of datasheet of the DS1307 chip, I noticed something weird:

"DC power is provided to the device on these pins. VCC is the +5V input. When 5V is applied within normal limits, the device is fully accessible and data can be written and read. When a 3V battery is connected to the device and VCC is below 1.25 x VBAT, reads and writes are inhibited. However, the timekeeping function continues unaffected by the lower input voltage. As VCC falls below VBAT the RAM and timekeeper are switched over to the external power supply (nominal 3.0V DC) at VBAT."

Does this mean that if I am powering the Grove RTC via the normal Grove Adapter which connects to the Pmod port of PYNQ and uses the 3.3 V source, I can not use 3.0 V coin cell battery anymore? 

Based on what I calculated, 1.25 * 3.0 V = 4.0 V and the 3.3 V VCC provided from the PYNQ is below the 4 V which inhibits the read and write process based on the datasheet. Obviously, if I make my VCC 5 V (which is kind of difficult in terms of wiring) or drop the VBAT below 2.5 V, this is not happening (I tested it). This requirement totally defeats the purpose of using Grove RTC with most of Xilinx FPGA boards. Because Pmods are supposed to be connected to the Pmod ports and not the 5 V source, and also there is no 2.5 V or lower voltage coin cell battery that I can use. So, I am kind of stuck at this point on how to use the Grove RTC in the normal way and have a backup battery as well. Any help is appreciated.

Thanks,
Mahdi

[JColvin]

Hi @Mahdi,

I get 3.75V from the 1.25V * Vbat calculation, but the issue you found remains regardless.

From what I am able to tell, your deduction is correct that you are not able to use both primary power (in this case 3.3V) and a backup battery at the same time on the Pynq board or other system boards that use 3.3V logic. I believe you are using the "normal precision" Grove RTC, but the normal precision value one seems to currently have support for Arduino and Raspberry PI, which I believe both use 5V logic (the high precision one doesn't have any supported platforms listed and doesn't seem to support battery powered based on the chip IC despite having a battery holder). So what you would need to do is boost the VCC voltage through external circuitry (such as a buck converter) to above 3.75 V (or 4V to be safe) in order for it to work correctly; naturally, this is not very convenient, but I am not sure if there is another alternative

Let me know if you have any questions about this.

Thanks,
JColvin