RM0319 datasheet(326/368 Pages) STMICROELECTRONICS

326 / 368 page

Power, reset and clock control

RM0319

326/368

Doc ID 022640 Rev 3

30.2

Transition states

When switching from one system operation state to another the system passes through

certain transition states as shwon in Figure 109

Figure 109. System control state machine transitions

30.2.1

XTAL control transition state, XTAL CTL

XTAL control transition state is used to initialize the crystal oscillator. While in this state, both

the system clocks and the system controller clock are driven from a low-frequency oscillator.

The system moves into the Switch to XTAL transition state when the crystal oscillator output

is stable. This is indicated when either the XTAL timeout defined in the XTAL control register

expires (when the XTALTIMEEN input is valid) or by the XTALON input being set to logic ‘1’.

30.2.2

Switch to XTAL transition state, SW TO XTAL

Switch to XTAL transition state is used to initiate the switching of the system clock source

from the slow speed oscillator to the crystal oscillator. The system moves into the SLOW

PLLSW

NORMAL

SW From

PLL

NORMAL

PLL SW

NORMAL

PLLON ||(PLLTIMEEN && PLL Timeout)

SW to

PLL

SLOW

SW From

XTAL

DOZE

XTAL

CTL

SW to

XTAL

SLOW&NORMAL

XTALSW

SLOW |

NORMAL

XTALON ||(XTALTIMEEN && XTAL Timeout)

XTAL SW

IRQ | FIQ

DOZE&SLOW&NORMAL

&STANDBYWFI

nPOR

SLEEP

PLL

CTL

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