HT48R068B_15 Datasheet, PDF (37/118 Pages) Holtek Semiconductor Inc – Enhanced I/O 8-Bit OTP MCU

HT48R068B/HT48R069B
Enhanced I/O 8-Bit OTP MCU
Mode Switching
The devices are switched between one mode and another using a combination of the CLKMOD bit
in the CTRL0 register and the HALT instruction. The CLKMOD bit chooses whether the system
runs in either  the Normal or Slow Mode by selecting the system clock to be sourced from either
a high or low frequency oscillator. The HALT instruction forces the system into either the Idle or
Sleep Mode, depending upon whether the LXT oscillator is running or not. The HALT  instruction
operates independently of the CLKMOD bit condition.
When a HALT instruction is executed and the LXT oscillator is not running, the system enters the
Sleep mode the following conditions exist:
• The system oscillator will stop running and the application program will stop at the "HALT"
instruction.
• The Data Memory contents and registers will maintain their present condition.
• The WDT will be cleared and resume counting if the WDT clock source is selected to come from
the LIRC oscillator. The WDT will stop if its clock source originates from the system clock.
• The I/O ports will maintain their present condition.
• In the status register, the Power Down flag, PDF, will be set and the Watchdog time-out flag, TO,
will be cleared.
Standby Current Considerations
As the main reason for entering the Idle/Sleep Mode is to keep the current consumption of the
MCU to as low a value as possible, perhaps only in the order of several micro-amps, there are
other considerations which must also be taken into account by the circuit designer if the power
consumption is to be minimised.
Special attention must be made to the I/O pins on the device. All high-impedance input pins must
be connected to either a fixed high or low level as any floating input pins could create internal
oscillations and result in increased current consumption. Care must also be taken with the loads,
which are connected to I/O pins, which are setup as outputs. These should be placed in a condition
in which minimum current is drawn or connected only to external circuits that do not draw current,
such as other CMOS inputs.
If the configuration options have enabled the Watchdog Timer internal oscillator LIRC then this
will continue to run when in the Idle/Sleep Mode and will thus consume some power. For power
sensitive applications it may be therefore preferable to use the system clock source for the Watchdog
Timer. The LXT, if configured for use, will also consume a limited amount of power, as it continues
to run when the device enters the Idle Mode. To keep the LXT power consumption to a minimum
level the LXTLP bit in the CTRL0 register, which controls the low power function, should be set
high.
Rev. 1.30
37
February 05, 2015