SAM3S8B Atmel Corporation, SAM3S8B Datasheet - Page 441

• Manufacturer Part Number: SAM3S8B
• Manufacturer: Atmel Corporation

25.0.4.1
25.0.4.2
11090A–ATARM–10-Feb-12
11090A–ATARM–10-Feb-12
Slow Clock RC Oscillator
Slow Clock Crystal 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).
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
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 25-2. 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
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 crys-
tal. 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 char-
acteristics section.
The programmer has to be sure to set the OSCBYPASS bit in the Supply Controller Mode Reg-
ister (SUPC_MR) and XTALSEL bit in the Supply Controller Control Register (SUPC_CR).
Figure
XIN32
25-2. More details are given in the section “DC Characteris-
32,768 Hz
Crystal
XOUT32
GND
SAM3S8/SD8
SAM3S8/SD8
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