BS2824 Holtek Semiconductor Inc., BS2824 Datasheet - Page 31

• Manufacturer Part Number: BS2824
• Description: Touch Key Mcu
• Manufacturer: Holtek Semiconductor Inc.

Microcontroller Operating Modes
Rev. 1.00
Microcontroller Mode Types and Selection
By using the LXT low frequency oscillator in combination with a high frequency oscillator, the
microcontroller can be selected to operate in a number of different modes. These Modes are Normal,
Slow, Idle and Sleep.
The higher frequency oscillators provide higher performance but carry with it the disadvantage of
higher power requirements, while the opposite is of course true for the lower frequency oscillators.
With the capability of dynamically switching between fast and slow oscillators, the device has the
flexibility to optimise the performance/power ratio, a feature especially important in power sensitive
portable applications.
For these devices, if the LXT oscillator is used then the internal RC oscillator, HIRC, must be used
as the high frequency oscillator. If the HXT or the ERC oscillator is chosen as the high frequency
system clock then the LXT oscillator cannot be used for sharing the same pins. The CLKMOD bit
in the CTRL0 register can be used to switch the system clock from the high speed HIRC oscillator
to the low speed LXT oscillator. When the HALT instruction is executed and the device enters the
Idle/Sleep Mode the LXT oscillator will always continue to run. For these devices the LXT crystal
is connected to the OSC1/OSC2 pins and LXT will always run (the LXTEN bit is not used).
Note that CLKMOD is only valid in HIRC+LXT oscillator configuration.
For these devices the LXT oscillator can run together with any of the high speed oscillators,
namely the HXT, ERC or the HIRC. The CLKMOD bit in the CTRL0 register can be used to
switch the system clock from the selected high speed oscillator to the low speed LXT oscillator.
When the HALT instruction is executed the LXT oscillator can be chosen to run or not using the
LXTEN bit in the CTRL2 register.
For all devices, when the system enters the Sleep or Idle Mode, the high frequency system clock
will always stop running. The accompanying tables shows the relationship between the CLKMOD
bit, the HALT instruction and the high/low frequency oscillators. The CLMOD bit can change
normal or Slow Mode.
BS2824, BS2854
BS2868
System Clock Configurations
31
BS2824/BS2854/BS2868
Touch Key MCU
January 27, 2010