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SAM4CP [DATASHEET]
43051E–ATPL–08/14
29.4.2 Slow Clock Crystal Oscillator
The Clock Generator integrates a 32768 Hz low-power oscillator. To use this oscillator, the XIN32 and XOUT32 pins
must be connected to a 32768 Hz crystal. Two external capacitors must be wired as shown in
Figure 29-2
. 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 29-2.
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 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) or the OSCSEL bit of the PMC Status Register
(PMC_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 the XTALSEL bit of SUPC_CR, 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 SUPC_MR and XTALSEL bit in the SUPC_CR.
XIN32
XOUT32
GND
32768 Hz
Crystal