Problems with powering Pmod Grove RTC using backup battery


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 (<LINK_TEXT text=“ … DS1307.pdf”></LINK_TEXT>), 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.




that means, when you use a 3v battery, you can not use 3.3v as the VCC, but 3.3v can still be used as a logical level.

And most boards of 3.3v as logic level provides a 5v pin. So you just need to connect the 5v pin to VCC pin of the grove by a jumper and then it can work.