Техническая Спецификация для Atmel Evaluation Kit AT91SAM9G25-EK AT91SAM9G25-EK
Модели
AT91SAM9G25-EK
177
SAM9G25 [DATASHEET]
11032C–ATARM–25-Jan-13
21.6
Main Clock Selection
The main clock can be generated either by an external 12 MHz crystal oscillator or by the on-chip 12 MHz RC oscillator.
This fast RC oscillator allows the processor to start or restart in a few microseconds when 12 MHz internal RC is selected.
This fast RC oscillator allows the processor to start or restart in a few microseconds when 12 MHz internal RC is selected.
The 12 MHz crystal oscillator can be bypassed by setting the bit MOSCXTBY to accept an external main clock on XIN.
Figure 21-4. Main Clock Selection
MOSCRCEN, MOSCXTEN, MOSCSEL and MOSCXTBY bits are located in the PMC Clock Generator Main Oscillator
Register (CKGR_MOR).
Register (CKGR_MOR).
After a VDDBU power on reset, the default configuration is MOSCRCEN = 1, MOSCXTEN = 0 and MOSCSEL = 0, the
12 MHz RC oscillator is started as Main clock.
12 MHz RC oscillator is started as Main clock.
21.6.1 Fast wake-up
To speed up the wake-up phase, the system boots on 12 MHz RC (Main Clock). This allows the user to perform system
configuration (PLL, DDR2, etc.) at 12 MHz instead of 32 kHz during 12 MHz oscillator start-up.
configuration (PLL, DDR2, etc.) at 12 MHz instead of 32 kHz during 12 MHz oscillator start-up.
Figure 21-5. PMC Startup
On Chip
12M RC OSC
Main Clock
Main Clock
Oscillator
MOSCXTEN
MOSCRCEN
MOSCSEL
XIN
XOUT
MOSCXTBY
Extern
a
l M
a
in Cock
M
a
in
Su
pply
POR o
u
tp
u
t
12 MHz RC
S
t
a
rt
u
p Time
W
a
it MO
S
CRC
S
= 1
S
y
s
tem
s
witche
s
on M
a
in Clock
to
s
peed-
u
p the
b
oot
S
y
s
tem i
s
r
u
nning
a
t 12 MHz
Extern
a
l o
s
cill
a
tor
i
s
s
t
a
rted for
b
etter
a
cc
u
r
a
cy
MO
S
CXTEN = 1
MO
S
C
S
EL = 0
S
y
s
tem
s
t
a
rt
s
on
3
2 kHz RC
RCEN = 1
O
O
S
C
3
2EN = 0
O
S
C
S
EL = 0
MO
S
CRCEN = 1
MO
S
CXTEN = 0
MO
S
C
S
EL = 0
PMC_MCKR = 1
Cry
s
t
a
l
S
t
a
rt
u
p Time
W
a
it MO
S
CXT
S
= 1
U
s
er
s
witche
s
on extern
a
l o
s
cill
a
tor
MO
S
C
S
EL=1
W
a
it while MO
S
C
S
EL
S
=1
S
y
s
tem i
s
r
u
nnning on 12 MHz Cry
s
t
a
l
PLL c
a
n
b
e
us
ed
12 MHz RC