SAM4CM_14 Datasheet, PDF (558/1177 Pages) ATMEL Corporation – Atmel | SMART ARM-based Flash MCU

29.4
Slow Clock
The Supply Controller embeds a slow clock generator that is supplied with the VDDBU power supply. As soon as
VDDBU 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).
29.4.1 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.
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
XIN32
XOUT32
32768 Hz
Crystal
GND
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 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 SUPC_MR and XTALSEL bit in SUPC_CR.
558
SAM4CM Series [DATASHEET]
Atmel-11203C-ATARM-SAM4CM32-SAM4CM16-SAM4CM8-Datasheet_06-Oct-14