Atmel SAM4S Series, Preliminary Data Sheet

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477
11100B–ATARM–31-Jul-12
SAM4S Series [Preliminary]
27.1.4 Slow Clock
The Supply Controller embeds a slow clock generator that is supplied with the VDDIO power
supply. As soon as the VDDIO is supplied, both the crystal oscillator and the embedded RC
oscillator 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 Slow Clock is generated either by the Slow Clock Crystal Oscillator or by the Slow Clock RC
Oscillator.
The selection between the RC or the crystal oscillator is made by writing the XTALSEL bit in the
Supply Controller Control Register (SUPC_CR).
27.1.4.1 Slow Clock RC Oscillator
By default, the Slow Clock RC Oscillator is enabled and selected. The user has to take into
account the possible drifts of the RC Oscillator. More details are given in the section “DC Char-
acteristics” of the product datasheet.
It can be disabled via the XTALSEL bit in the Supply Controller Control Register (SUPC_CR).
27.1.4.2 Slow Clock Crystal Oscillator
The Clock Generator integrates a 32,768 Hz low-power oscillator. In order to use this oscillator,
the XIN32 and XOUT32 pins must be connected to a 32,768 Hz crystal. Two external capacitors
must be wired as shown in
Figure 27-2 . More details are given in the section “DC Characteris-
tics” of the product datasheet.
Note that the user is not obliged to use the Slow Clock Crystal and can use the RC oscillator
instead.
Figure 27-2. Typical Slow Clock Crystal Oscillator Connection
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 configures the PIO
lines multiplexed with XIN32 and XOUT32 to be driven by the oscillator, then enables 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) tracks the
oscillator frequency downstream. It must be read in order to be informed when the switch
sequence, initiated when a new value is written in MOSCSEL bit of CKGR_MOR, is done.
Coming back on the RC oscillator is only possible by shutting down the VDDIO power supply. If
the user does not need the crystal oscillator, the XIN32 and XOUT32 pins can be left uncon-
nected since by default the XIN32 and XOUT32 system I/O pins are in PIO input mode with pull-
up after reset.
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 XIN32. The input characteristics of
XIN32 XOUT32 GND
32,768 Hz
Crystal