STR910FAM32 datasheet(23/99 Pages) STMICROELECTRONICS

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STR91xFAx32 STR91xFAx42 STR91xFAx44

Functional overview

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3.11

Flexible power management

The STR91xFA offers configurable and flexible power management control that allows the

user to choose the best power option to fit the application. Power consumption can be

dynamically managed by firmware and hardware to match the system’s requirements.

Power management is provided via clock control to the CPU and individual peripherals.

Clocks to the CPU and peripherals can be individually divided and gated off as needed. In

addition to individual clock divisors, the CCU master clock source going to the CPU, AHB,

APB, EMI, and FMI can be divided dynamically by as much as 1024 for low power operation.

Additionally, the CCU may switch its input to the 32.768 kHz RTC clock at any time for low

power.

The STR91xFA supports the following three global power control modes:

●

Run Mode: All clocks are on with option to gate individual clocks off via clock mask

registers.

●

Idle Mode: CPU and FMI clocks are off until an interrupt, reset, or wake-up occurs.

Pre-configured clock mask registers selectively allow individual peripheral clocks to

continue run during Idle Mode.

●

Sleep Mode: All clocks off except RTC clock. Wake up unit remains powered, PLL is

forced off.

A special mode is used when JTAG debug is active which never gates off any clocks even if

the CPU enters Idle or Sleep mode.

3.11.1

Run mode

This is the default mode after any reset occurs. Firmware can gate off or scale any individual

clock. Also available is a special Interrupt Mode which allows the CPU to automatically run

full speed during an interrupt service and return back to the selected CPU clock divisor rate

when the interrupt has been serviced. The advantage here is that the CPU can run at a very

low frequency to conserve power until a periodic wake-up event or an asynchronous

interrupt occurs at which time the CPU runs full speed immediately.

3.11.2

Idle mode

In this mode the CPU suspends code execution and the CPU and FMI clocks are turned off

immediately after firmware sets the Idle Bit. Various peripherals continue to run based on

the settings of the mask registers that exist just prior to entering Idle Mode. There are 3

ways to exit Idle Mode and return to Run Mode:

●

Any reset (external reset pin, watchdog, low-voltage, power-up, JTAG debug command)

●

Any interrupt (external, internal peripheral, RTC alarm or interval)

●

Input from wake-up unit on GPIO pins

Note:

It is possible to remain in Idle Mode for the majority of the time and the RTC can be

programmed to periodically wake up to perform a brief task or check status.

3.11.3

Sleep mode

In this mode all clock circuits except the RTC are turned off and main oscillator input pins

X1_CPU and X2_CPU are disabled. The RTC clock is required for the CPU to exit Sleep

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