HT46R92 datasheet(40/61 Pages) HOLTEK

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HT46R92

Rev. 1.10

November 5, 2008

External RTC Oscillator

When the microcontroller enters the Power Down Mode,

the system clock is switched off to stop microcontroller

activity and to conserve power. However, in many

microcontroller applications it may be necessary to keep

some internal functions such as timers operational even

when the microcontroller is in the Power Down Mode. To

do this, a 32768Hz oscillator, also known as the Real

Time Clock or RTC oscillator, is provided. To implement

this clock, the OSC3 and OSC4 pins should be con-

nected to a 32768Hz crystal. However, for some crys-

tals, to ensure oscillation and accurate frequency

generation, it may be necessary to add two small value

external capacitors, C1 and C2. The exact values of C1

and C2 should be selected in consultation with the crys-

tal or resonator manufacturer

¢s specification. The exter-

nal parallel feedback resistor, Rp, is normally not

required but in some cases may be needed to assist

with oscillation start up. The MODS bit in the MODE reg-

ister is used to select whether the external 32768Hz os-

cillator or the External Crystal/External RC is used as

the system oscillator. Using the slower 32768Hz oscilla-

tor as the system oscillator will of course use less power

and is known as the Slow Mode.

Internal Ca, Cb, Rf Typical Values @ 5V, 25

°C

TBD

RTC Oscillator Internal Component Values

RTC Oscillator C1 and C2 Values

Crystal Frequency

32768Hz

TBD

Note:

1. C1 and C2 values are for guidance only.

2. CL is the crystal manufacturer specified

load capacitor value.

32768 Hz Crystal Recommended Capacitor Values

When the system enters the Power Down Mode, the

32768Hz oscillator will keep running and if it is selected

as the Timer and Watchdog Timer source clock, will also

keep these functions operational.

During power up there is a time delay associated with

the RTC oscillator, waiting for it to start up. The QOSC

bit in the MODE register, is provided to give a quick

start-up function and can be used to minimise this delay.

During a power up condition, this bit will be cleared to 0

which will initiate the RTC oscillator quick start-up func-

tion. However, as there is additional power consumption

associated with this quick start-up function, to reduce

power consumption after start up takes place, it is rec-

ommended that the application program should set the

QOSC bit high about 2 seconds after power on. It should

be noted that, no matter what condition the QOSC bit is

set to, the RTC oscillator will always function normally,

only there is more power consumption 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.

O S C 3

O S C 4

R f

T o i n t e r n a l

c i r c u i t s

C 1

C 2

C a

C b

R p

N o t e : 1 . R p i s n o r m a l l y n o t r e q u i r e d .

2 . A l t h o u g h n o t s h o w n O S C 3 / O S C 4 p i n s h a v e a p a r a s i t i c

c a p a c i t a n c e o f a r o u n d 7 p F .

3 2 7 6 8 H z

Internal RC Oscillator + External RTC Oscillator

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