Datasheet

521
SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
27.4.1 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 Characteristics” of the product datasheet.
It can be disabled via the XTALSEL bit in the Supply Controller Control Register (SUPC_CR).
27.4.2 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 Characteristics” 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 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 VDDBU power supply. If the user does not
need the crystal oscillator, the XIN32 and XOUT32 pins can 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 XIN32. The input characteristics of the XIN32 pin are given in the
product electrical characteristics section. In order to set the bypass mode, the OSCBYPASS bit of the Supply
Controller Mode Register (SUPC_MR) needs to be set at 1.
The user can set the Slow Clock 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 the XIN32 pin under these
conditions are given in the product electrical characteristics section.
The programmer has to be sure to set the OSCBYPASS bit in the Supply Controller Mode Register (SUPC_MR)
and XTALSEL bit in the Supply Controller Control Register (SUPC_CR).
XIN32 XOUT32 GND
32,768 Hz
Crystal