Kontron COMe Starterkit Eval T2 - Manual (Page 134)

COM Express Interfaces
2.22.3. RTC Battery Implementation
The Real Time Clock (RTC) is responsible for maintaining the time and date even when the COM
Express Module is not connected to a main power supply. Usually a +3V lithium battery cell is
used to supply the internal RTC of the Module. The COM Express Specification defines an extra
power pin 'VCC_RTC', which connects the RTC of the Module to the external battery. The
specified input voltage range of the battery is defined between +2.0V and +3.0V. The signal
'VCC_RTC' can be found on the Module's connector row A pin A47.
To implement the RTC Battery according to the Underwriters Laboratories Inc ® (UL) guidelines
(UL 1642), battery cells must be protected against a reverse current going to the cell. UL 1642
requires two blocking components (such as a pair of diodes) or a resistor and a diode. The
resistor has to be large enough to prevent a current of more than one third of the battery's
maximum allowed reverse charging current. An alternative to a pair of diodes are specialty ICs
from companies such as Maxim or Dallas Semiconductor that have a UL1642 approval and
perform the required blocking function.
The resistor and diode option is shown in Figure 53 below.
A possible drawback of this circuitry is that the battery voltage monitoring result displayed by the
COM Express Module will be inaccurate due to current leakage on the Module side. When the
system is running, this current leakage loads the capacitor of the battery circuitry. This leads to a
higher voltage on the signal pin 'VCC_RTC' and therefore produces inaccurate monitoring
results.
Figure 53: RTC Battery Circuitry with Serial Schottky Diode
2.22.3.1. RTC Battery Lifetime
The RTC battery lifetime determines the time interval between system battery replacement
cycles. Current leakage from the RTC battery circuitry on the Carrier Board is a serious issue
and must be considered during the system design phase. The current leakage will influence the
RTC battery lifetime and must be factored in when a specific life expectancy of the system
battery is being defined.
In order to accurately measure the value of the RTC current, it should be measured when the
complete system is disconnected from primary power.
For information about the power consumption of the RTC circuit, refer to the Module's user's
guide.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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CEX VCC_RTC
R176
470
D100
BAT54S
B
Battery Holder