RTC Behavior When Supplying VCC and Vbat From the Same Source

[jake11212]

I'm designing a PCB that uses the nrf52832 with an external RTC, the DS1339 (https://datasheets.maximintegrated.com/en/ds/DS1339-DS1339U.pdf).  I want to use a single 3.3V coin cell battery to power both the nrf52832 as well as the DS1339.  With this typical operating circuit (below) I have pins 3 and 8 both connected to VDD_nRF which is supplied by the 3.3V battery.  This led to a few questions:

1.) Will the DS1339 ever go into battery mode given that it will always have the 3.3V battery supply?  Do I need any sort of filtering network to help with power timing if/when the switch happens?

2.) Is there a large difference in power consumption between battery mode and regular operating?  Is this significant compared to the power consumption of the nrf52832?  I couldn't find any information about power consumption for the RTC

3.) If the power consumption of the RTC in its regular (non-battery powered) state is significant, is there a way through hardware/software to cause the DS1339 to only see 3.3V on VCC when the nrf52832 is not in sleep mode so that it will switch to battery mode when the nrf52832 is in sleep mode?

[StefXinet]

Dear jake11212,

jake11212 said:

1.) Will the DS1339 ever go into battery mode given that it will always have the 3.3V battery supply?  Do I need any sort of filtering network to help with power timing if/when the switch happens?

jake11212 said:

3.) If the power consumption of the RTC in its regular (non-battery powered) state is significant, is there a way through hardware/software to cause the DS1339 to only see 3.3V on VCC when the nrf52832 is not in sleep mode so that it will switch to battery mode when the nrf52832 is in sleep mode?

you could use a P-MOS with the gate connected to a PIN of the nRF52  to manage power supply to the PIN 8 of DS1339. The PIN 3 should be connected to the battery, in this way you can switch from normal mode and sleep mode.

jake11212 said:

2.) Is there a large difference in power consumption between battery mode and regular operating?  Is this significant compared to the power consumption of the nrf52832?  I couldn't find any information about power consumption for the RTC

The DS1339 power consumption compared to nRF52 depends on what the nRF52 is doing: if you are using BLE or the radio part it consumes lot of power; if nRF52 is sleeping, it is comparable.

bye

[jake11212]

Thank you!  Is there a specific pin that would work well for this?  Which pins will have 3.3V when the nrf52832 is not in sleep mode?  I tested the voltages but none of the pins go high during run mode and low during sleep mode, so I think the only way to do this would be to have some sort of circuit to invert the logic.