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Figure 10.3. Backup power domain overview
EFM3 2
BUCTRL_EN
Main power BOD
BUCTRL_STATEN
BU_STAT
BUINACT_PWRCON /
BUACT_PWRCON
VDD_DREG
Main
power ⁺
supply ^-
Main power
Main dom ain
reset
PWRCONF_PWRRES
BUBODBUVIN
Backup power
BU_VIN
+ ^Backup
- ^power
supply
BUBODVDDDREG
BUBODUNREG
Backup regulator
BUBODREG
SW reset
Backup dom ain
BURTC
STRONG
MEDIUM
WEAK
BU_VOUT
PWRCONF_VOUTx x x
BURTC
Wake-Up
512 byte
retention
POR
EXT
BOD
WDOG
Wake-up
controller
RESETn
Main power OK
EM4 pin Wake-Up
When in backup mode, available functionality is the same as the functionality available in EM4. Refer
to Section 10.3.4.10 (p. 115) for further details.
10.3.4.2 Brown out detectors
The backup power domain functionality utilizes four brown-out detectors, BODs. One senses the main
power supply, one senses the backup power supply, one senses the unregulated selected power supply
(main or backup, depending on mode), and one BOD senses the regulated power supply. The bits
BUBODVDDDREG ,BUBODBUVIN, BUBODUNREG, and BUBODREG in the RSTCAUSE register in
the RMU are set when the associated BOD triggers. The locations of the Backup BODs are indicated in
Figure 10.3 (p. 112) . A brown out on the main power supply will trigger a switch to the backup power
supply if the backup functionality is enabled and the BOD sensing on the backup power supply has not
triggered. The two other BODs are used for error indication and will only set the bits in RMU_RSTCAUSE
if they are triggered.
A reset from backup mode on BUBODUNREG brown-out can also be triggered if BUMODEBODEN in
EMU_BUCTRL is set. This will cause the device to switch back to the main power supply regardless
of whether this is valid or not. Set this bit to make sure the device always asssume a known condition
when the backup voltage drops below the operating limits.
10.3.4.3 Entering backup mode
To be able to enter backup mode, the EN bit in EMU_BUCTRL has to be set. The BURDY interrupt
flag will be set as soon as the backup sensing module is operational. Status of the backup functionality
is also available in the BURDY flag in the EMU_STATUS register. The BU_VIN pin also needs to be
enabled. This is done by setting the BUVINPEN bit in EMU_ROUTE. To enter backup mode, the voltage
on VDD_DREG has to drop below the programmable threshold of the BOD sensing on this power. This
threshold is programmed using BUENRANGE and BUENTHRES in EMU_BUINACT. BUENRANGE
2013-05-08 - Wonder Gecko Family - d0233_Rev0.50
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