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ATSAM4S-XPLD
451
SAM4S [DATASHEET]
11100E–ATARM–24-Jul-13
28.1.4.1 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.
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).
28.1.4.2 Slow Clock Crystal Oscillator
The Clock Generator integrates a 32768 Hz low-power oscillator. In order 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
XIN32 and XOUT32 pins must be connected to a 32768 Hz crystal. Two external capacitors must be
wired as shown in
product datasheet.
Note that the user is not obliged to use the Slow Clock Crystal and can use the RC oscillator instead.
Figure 28-2. 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) 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.
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) 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.
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.
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.
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).
(SUPC_MR) and XTALSEL bit in the Supply Controller Control Register (SUPC_CR).
XIN32
XOUT32
GND
32768 Hz
Crystal