User's Manual l MBa8Xx UM 0100 l © 2020, TQ-Systems GmbH
Page 17
3.6
RTC backup
The TQMa8Xx has an optional RTC and a voltage rail for standby functions. This is supplied via pin LICELL.
On the MBa8Xx there are two possibilities to supply the LICELL input
:
•
CR2032-Battery holder
•
2-pin header for alternative connection of an external voltage or batteries
The maximum input voltage at X7 is described in the TQMa8Xx User’s Manual.
TQMa8Xx
LICELL
CR2032
Battery socket
(X6)
2-pin Header
(X7)
U
F
<0.1 V
Figure 6:
Block diagram RTC backup
Table 15:
Pinout RTC backup (X7)
Pin
Signal
Level
Remark
1
V_LICELL
3 V
Supply voltage for RTC on TQMa8Xx
2
GND
0 V
Ground
To meet regulatory requirements for protection against accidental battery charging, a 1 kΩ resistor and diode is connected in
series between X6/X7 and the LICELL input of the TQMa8Xx.
The diode has a very low forward voltage (<0.1 V) in order not to unnecessarily reduce the battery runtime.
If it is necessary to reduce the forward voltage to 0 V for test purposes, the protective diode can be removed according to Table
16 and a corresponding resistor can be fitted.
Table 16:
Assembly option RTC-Backup
Reverse current protection
V2106
R2110
Remark
Active
BAT54
NP
Default
Not active
NP
0 Ω
Note: 1 kΩ in series is still present in this case
Attention: Loss of reverse current protection
The reverse current protection is lost when the protective diode is replaced with a resistor!
This is especially critical for TQMa8Xx with external RTC, since here the 3.3 V supply of the TQMa8Xx
in ON mode
drives directly into the RTC backup battery!
11:
Only one of the two options may be used at a time!
12:
TQMa8Xx is supplied at V_3V3_IN.
