HT46R01 datasheet(42/60 Pages) HOLTEK

42 / 60 page

HT46R01/HT46R02/HT46R03

Rev. 1.00

September 21, 2007

For applications using the RTC oscillator, the system

clock can be chosen to be either the Internal System RC

Oscillator or the RTC oscillator itself. This selection is

made using the CLKMOD bit in the CTRL0 register. If

this bit is set high then the 32768Hz external crystal will

also provide the system clock source. If the bit is low

then the system clock source will be the Internal RC Os-

cillator. When the system enters the Power Down Mode,

the system clock, irrespective of whether the CLKMOD

bit has selected the RTC or Internal RC Oscillator as its

source, will always stop running. The following table

shows the relationship between the CLKMOD bit and

the various oscillators.

Operating

Mode

CLKMOD

Bit

Internal

RC Osc.

RTC

System

Clock

Normal

Running

RC Oscillator

Off

32768Hz

Power

Down

Off

Stopped

During power up there is a time delay associated with

the RTC oscillator waiting for it to start up. Bit 1 of the

CTRL0 register, known as the QOSC bit, is provided to

give a quick start-up function and can be used to mini-

mize this delay. During a power up condition, this bit will

be cleared to 0 which will initiate the RTC oscillator quick

start-up function. However, as there is additional power

consumption associated with this quick start-up func-

tion, to reduce power consumption after start up takes

place, it is recommended that the application program

should set the QOSC bit high about 2 seconds after

power on. It should be noted that, no matter what condi-

tion the QOSC bit is set to, the RTC oscillator will always

function normally, only there is more power consump-

tion associated with the quick start-up function.

Watchdog Timer Oscillator

The WDT oscillator is a fully self-contained free running

on-chip RC oscillator with a typical period of 65

msat5V

requiring no external components. When the device en-

ters the Power Down Mode, the system clock will stop

running but the WDT oscillator continues to free-run and

to keep the watchdog active. However, to preserve

power in certain applications the WDT oscillator can be

disabled via a configuration option.

Power Down Mode and Wake-up

Power Down Mode

All of the Holtek microcontrollers have the ability to enter

a Power Down Mode, also sometimes known as the

HALT Mode or Sleep Mode. When the device enters this

mode, the normal operating current, will be reduced to

an extremely low standby current level. This occurs be-

cause when the device enters the Power Down Mode,

the system oscillator is stopped which reduces the

power consumption to extremely low levels, however,

as the device maintains its present internal condition, it

can be woken up at a later stage and continue running,

without requiring a full reset. This feature is extremely

important in application areas where the MCU must

have its power supply constantly maintained to keep the

device in a known condition but where the power supply

capacity is limited such as in battery applications.

Entering the Power Down Mode

There is only one way for the device to enter the Power

Down Mode and that is to execute the

²HALT² instruc-

tion in the application program. When this instruction is

executed, the following will occur:

· The system oscillator will stop running and the appli-

cation program will stop at the

²HALT² instruction.

· If the RTC oscillator configuration option is enabled

then the RTC clock will keep running.

· 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 WDT

or RTC 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 Power Down 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 con-

nected 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 con-

nected only to external circuits that do not draw current,

such as other CMOS inputs.

· I n t e r n a l R C S y s t e m O s c i l l a t o r

w i t h E x t e r n a l R T C O s c i l l a t o r

· 3 2 7 6 8 H z C r y s t a l C o n n e c t e d

t o P A 5 / P A 6

I n t e r n a l R C

O s c i l l a t o r

O S C 1

O S C 2

1 0 p F

P A 5

P A 6

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