Atmel ARM-Based Evaluation Kit AT91SAM9N12-EK AT91SAM9N12-EK Data Sheet
Product codes
AT91SAM9N12-EK
169
SAM9N12/SAM9CN11/SAM9CN12 [DATASHEET]
11063K–ATARM–05-Nov-13
21.4
Slow Clock Selection
The slow clock can be generated either by an external 32,768 Hz crystal or by the on-chip 32 kHz RC oscillator. The
32,768 Hz crystal oscillator can be bypassed by setting the bit OSC32BYP to accept an external slow clock on XIN32.
32,768 Hz crystal oscillator can be bypassed by setting the bit OSC32BYP to accept an external slow clock on XIN32.
The internal 32 kHz RC oscillator and the 32,768 Hz oscillator can be enabled by setting to 1, respectively, RCEN bit and
OSC32EN bit in the System Controller user interface. The OSCSEL command selects the slow clock source.
OSC32EN bit in the System Controller user interface. The OSCSEL command selects the slow clock source.
Figure 21-2. Slow Clock
RCEN, OSC32EN,OSCSEL and OSC32BYP bits are located in the Slow Clock Control Register (SCKCR) located at
address 0xFFFFFE50 in the backed up part of the System Controller and so are preserved while VDDBU is present.
address 0xFFFFFE50 in the backed up part of the System Controller and so are preserved while VDDBU is present.
After a VDDBU power on reset, the default configuration is RCEN = 1, OSC32EN = 0 and OSCSEL = 0, BYPASS = 0,
allowing the system to start on the internal 32 kHz RC oscillator.
allowing the system to start on the internal 32 kHz RC oscillator.
The programmer controls the slow clock switching by software and so must take precautions during the switching phase.
21.4.1 Switch from Internal 32 kHz RC Oscillator to the 32,768 Hz Crystal
To switch from internal 32 kHz RC oscillator to the 32,768 Hz crystal, the programmer must execute the following
sequence:
sequence:
Switch the master clock to a source different from slow clock (PLL or Main Oscillator) through the Power
Management Controller.
Management Controller.
Enable the 32,768 Hz oscillator by setting the bit OSC32EN to 1.
Wait 32,768 Hz Startup Time for clock stabilization (software loop).
Switch from internal 32 kHz RC to 32,768 Hz oscillator by setting the bit OSCSEL to 1.
Wait 5 slow clock cycles for internal resynchronization.
Disable the 32 kHz RC oscillator by setting the bit RCEN to 0.
Switch the master clock back to the slow clock domain
On Chip
RC OSC
RC OSC
Slow Clock
SLCK
SLCK
XIN32
XOUT32
Slow Clock
Oscillator
OSC32EN
RCEN
OSCSEL
OSC32BYP