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1745D–ATARM–04-Nov-05
AT91M55800A
12.8 First Start-up Sequence
At initial startup, or after VDDBU has been disconnected, the battery-supplied logic must be
initialized.
The Battery Backup Reset sets the following default state:
• Shut-down Logic
Initialized in the Wake-up state (or Non Alarm)
• The Power Mode Register
Shut-down defines SHDN as level 0 (SHDALS = 1)
Wake-up defines SHDN as tri-state (WKACKS = 0)
• The Real-time Clock Configuration and Data Registers
A simple RC network can be used as a power-on reset for the battery supply.
The pin SHDN is tri-stated by default. An external resistor must hold the main power supply
shut-down pin in the inactive state. The shut-down logic can be programmed with the correct
active level of the power supply shut-down input during the first start-up sequence.
The first time the system is powered up, the SHDN pin is tri-stated because different power
supplies use different logic levels for their shut-down input signals. To minimize backup bat-
tery power consumption, there is no internal pull-up or pull-down on this signal.
If the power supply needs a logic level on its shut-down input in order to start the main power
supply then an external “Force Start Up” jumper is required to provide this level.
The jumper provides the necessary level on the SHDN to maintain the power supply when the
AT91 boots, and it can be removed until the next loss of battery power.