AT91SAM7L128PRE Datasheet, PDF (105/564 Pages) ATMEL Corporation – numberHigh-performance 32-bit RISC Architecture

AT91SAM7L128/64 Preliminary
17.3 Supply Controller Functional Description
17.3.1
Supply Controller Overview
The Supply Controller controls the power supplies of each section of the product:
• The Backup, including the Supply Controller, a part of the Reset Controller and the Slow
Clock switcher
• The backup SRAM
• The Clock, including the Real Time Clock
• The Flash Memory
• The CORE, including the other part of the Reset Controller, the Processor and the
Peripherals
• the LCD controller, the charge pump and the LCD voltage regulator
The Supply Controller has its own reset circuitry and is clocked by the 32 kHz Slow clock
generator.
The reset circuitry is based on the NRSTB pin, a zero-power power-on reset cell and a brownout
detector cell. The zero-power power-on reset allows the Supply Controller to start properly, while
the software-programmable Brownout Detector detects either a battery discharge or main volt-
age loss.
The Slow Clock generator is based on a 32 kHz crystal oscillator and an embedded 32 kHz RC
oscillator. The Slow Clock defaults to the RC oscillator, but the software can enable the crystal
oscillator and select it as the Slow Clock source.
The Supply Controller starts up the device by sequentially enabling the internal power switches
and the Voltage Regulator, then generates the proper reset signals to the core power supply.
It also sets the system in different low power modes and wakes it up from a wide range of
events.
17.3.2
Slow Clock Generator
The Supply Controller embeds a slow clock generator that is supplied with the backup power
supply. As soon as FWUP is asserted, both the crystal oscillator and the embedded RC oscilla-
tor are powered up, but only the embedded RC oscillator is enabled. This allows the slow clock
to be valid in a short time (about 100 µs).
The user can select the crystal oscillator to be the source of the slow clock, as it provides a more
accurate frequency. The command is made by writing the Supply Controller Control Register,
SUPC_CR, with the XTALSEL bit at 1. This results in a sequence which first enables the crystal
oscillator, then waits for 32,768 slow clock cycles, then switches the slow clock on the output of
the crystal oscillator and then disables the RC oscillator to save power. The switch of the slow
clock source is glitch free. The OSCSEL bit of the Supply Controller Status Register, SUPC_SR,
allows knowing when the switch sequence is done.
Coming back on the RC oscillator is only possible by shutting down the backup power supply.
If the user does not need the crystal oscillator, the XIN and XOUT pins should be left
unconnected.
The user can also set the crystal oscillator in bypass mode instead of connecting a crystal. In
this case, the user has to provide the external clock signal on the XIN. The input characteristics
of the XIN pin are given in the product electrical characteristics section. In order to set the
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