134 RabbitCore RCM3365/RCM3375A lithium battery with a nominal voltage of 3 V and a minimum capacity of 165 mA·h isrecommended. A lithium battery is strongly recommended because of its nearly constantnominal voltage over most of its life.The drain on the battery by the RCM3365/RCM3375 is typically 6 μA when no other poweris supplied. If a 165 mA·h battery is used, the battery can last about 3 years:The RCM3365/RCM3375 module does not drain the battery while it is powered upnormally.Cycle the main power off/on on the RCM3365/RCM3375 after you install a backup batteryfor the first time, and whenever you replace the battery. This step will minimize the currentdrawn by the real-time clock oscillator circuit from the backup battery should theRCM3365/RCM3375 experience a loss of main power.NOTE: Remember to cycle the main power off/on any time the RCM3365/RCM3375 isremoved from the Prototyping Board or motherboard since that is where the backupbattery would be located.Rabbit’s Technical Note TN235, External 32.768 kHz Oscillator Circuits, provides addi-tional information about the current draw by the real-time clock oscillator circuit.D.1.2 Battery-Backup CircuitFigure D-2 shows the battery-backup circuit.Figure D-2. RCM33365/RCM3375 Backup Battery CircuitThe battery-backup circuit serves three purposes:• It reduces the battery voltage to the SRAM and to the real-time clock, thereby limitingthe current consumed by the real-time clock and lengthening the battery life.• It ensures that current can flow only out of the battery to prevent charging the battery.• A voltage, VOSC, is supplied to U1, which keeps the 32.768 kHz oscillator workingwhen the voltage begins to drop.165 mA·h6 μA------------------------ 3.1 years.=1/#%')+1 8"9 0 - ()(1!# $#''&#''5%#'5
