RM0319 datasheet(321/368 Pages) STMICROELECTRONICS

321 / 368 page

RM0319

Power, reset and clock control

Doc ID 022640 Rev 3

321/368

Power, reset and clock control

This chapter provides a functional description of the system power, reset and clock control

features.

For details on programming the related registers, refer to the System Controller (SC) and

System Configuration registers (MISC) sections of the following companion document:

●

RM0321: SPEAr320S address map and registers

30.1

System control state machine

The system control state machine manages power consumption by controlling the clock

sources of the system clock (SYS_CLK). It has four system operation states:

●

SLEEP

●

DOZE (reset state)

●

SLOW

●

NORMAL

Switching between system operation states is software-controlled except for SLEEP to

DOZE which is activated only by a hardware event.

Three selections are mainly available:

●

Main oscillator: directly 24 MHz or its ratio (1:2, 1:4, 1:16 or 1:32)

●

RTC oscillator: if present it is 32.768 kHz

●

PLL1 Frequency: generated from the main oscillator

The state machine is controlled using three mode control bits (ModeCtrl[2:0]) in the system

control register (SCCTRL), which define the required system operating mode. The mode

control bits control the modes:

●

1xx If the most significant bit (MSB) is set then the system moves into NORMAL mode.

●

01x If the MSB is not set and the next MSB is set then the system moves into SLOW

mode.

●

001 If only the least significant bit (LSB) is set then the system moves into DOZE mode.

●

000 If none of the mode control bits are set the system moves into SLEEP mode.

When the required operating mode has been defined by software writing to the

ModeCtrl[2:0] bits in the system control register(SCCTRL) the state machine moves to the

required operating mode without further software interaction.

The current system mode can read by software from the ModeStatus[3:0] bits in the

SCCTRL register.

If the nPOR input is activated, the state machine and the required operating mode in the

system control register are set to DOZE. If the PRESETn input is activated, the system

mode control state machine does not change mode but the required operating mode is set

to DOZE in the system control register.

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