Hardware Description
11
• PLLB
• Programmable Clocks
The Main Oscillator frequency can be measured by using the PMC Main Clock Frequency
register. The SLCK is used as reference for the measurement.
4.11.2. Power Management
Using power management can dramatically reduce the power consumption of an
Embedded Device. Via the PMC various clocks can be disabled or their speed can be
reduced:
• stopping the PLLs (PLLA and / or PLLB)
• stopping the clocks of the various peripherals
• reducing the clock rates of peripherals, especially by changing MCK.
The PMC supports the following power-saving features: Idle mode and power-down mode.
Please note that not every operating system supports these modes.
•
Idle Mode.
In idle mode, the processor clock will be re-enabled by any interrupt. The
peripherals, however, are only able to generate an interrupt if they still have a clock, so
care has to be taken as to when a peripheral can be powered down.
•
Power-down Mode.
In many cases a system waits for a user action or some other
rare event. In such a case, it is possible to change MCK to SLCK. Any external event
which changes the state on peripheral pins (not the USB) can then be detected by the
PIO controller or the AIC.
It should also be taken into account that when a PLL is stopped it will take some time to
restart it. Changing the PLL frequencies or stopping them can therefore be done only
at a moderate rate. If short reaction times are required, this is not a choice.
Additionally, the following measures can reduce power consumption considerably:
• switching off the TFT supply voltage
• putting peripheral chips like Ethernet controller and / or PHY or serial driver devices
in power down mode
• putting the SDRAM into self-refresh mode
4.12. Real-time Timer (RTT)
The Real-time Timer is a 32-bit counter combined with a 16-bit prescaler running at Slow
Clock (SLCK = 32768 Hz). As the RTT keeps running if only the backup supply voltage is
available, it is used as a Real-time clock.
The RTT can generate an interrupt every time the prescaler rolls over. Usually the RTT is
configured to generate an interrupt every second, so the prescaler will be programmed
with the value 7FFFh.