Battery low detect

AN3060

14/23

Doc ID 16253 Rev 2

Periodic checking

The battery is checked every time the RTC powers up from backup mode, and every time

the clock rolls over at midnight (when not in backup mode). During the rollover check, the

battery is loaded to ensure a valid reading.

Power-up check

When an RTC has been in backup mode, its battery has already been loaded and thus a

load is not needed for the battery check. Batteries have a slow enough recovery time that

their outputs will not change quickly so it is not necessary to load them during the power-up

check. Nevertheless, a few devices - the M41T82, M41T83, M41ST87W and M41T93 -

include loading during the power-up check.

Detecting no battery - open circuit

Some applications call for being able to determine that no battery is installed. Ideally, a no

floated - the comparator output will be indeterminate. In order to reliably detect a no battery

condition, a load needs to be applied to the battery node prior to checking it. A resistor

applied across the battery terminals (or between B+ and ground in the case of devices with

no B– pin) is sufficient load for this purpose. This is impractical in the field, but can be useful

in the lab during development.

Using rollover to load the battery

Since the RTC applies a load to the battery during the midnight rollover check, it is possible

for applications to invoke the load by changing the time to near midnight, letting the clock

rollover, then setting the time back to its previous setting plus some delta to account for the

time spent waiting for rollover. This can be helpful during factory test to check that a battery

has been installed.

For example, if it's 17:53:27, the user would store that value, set the clock to 23:59:59, wait

two seconds, read the BL bit, then set the time back to 17:53:29 (17:53:27 + 2 seconds). In

this manner, the battery will be loaded during the check and thus a no battery condition will

be reliably read as battery low.

As stated above, some RTCs do load the clock during power-up, and thus a missing battery

can be detected by simply power-cycling the device.