
425
SAM G51 [DATASHEET]
11209C–ATARM–20-Dec-13
22.4
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 s).
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).
22.4.1 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 SUPC_CR.
22.4.2 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 22-2. 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 22-2.
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 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) 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 VDDIO power supply. If the user does not need
the crystal oscillator, the XIN32 and XOUT32 pins can be left unconnected 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 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 SUPC_MR and XTALSEL bit in SUPC_CR.
XIN32
XOUT32
GND
32768 Hz
Crystal