HT45R37V datasheet(67/90 Pages) HOLTEK

67 / 90 page

HT45R37V

Rev. 1.00

October 20, 2009

Internal 32K_INT Oscillator

When the device enters the Sleep, Slow or Idle Mode, its

high frequency clock is switched off to reduce activity

and to conserve power. However, in many

microcontroller applications it may be necessary to keep

some internal functions operational even when the

microcontroller is in the Power-down mode. To do this,

the device has a 32K_INT oscillator, which is a fully inte-

grated free running RC oscillator with a typical period of

31.2

ms at 5V, requiring no external components. It is se-

lected via configuration option.

External 32768Hz Oscillator

With a function similar to the internal 32K-INT 32KHz os-

cillator, that is to keep some device functions opera-

tional during power down, this device also has an

external 32768Hz oscillator. This clock source has a

fixed frequency of 32768Hz and requires a 32768Hz

crystal to be connected between pins OSC3 and OSC4.

The external resistor and capacitor components con-

nected to the 32768Hz crystal are not necessary to pro-

vide oscillation. For applications where precise

frequencies are essential, these components may be

required to provide frequency compensation due to dif-

ferent crystal manufacturing tolerances.

A configuration option selects whether the external

32768Hz oscillator or the internal 32K_INT oscillator is

selected. Selecting low frequency oscillators for use as

a system oscillator is implmented using bits in the

CLKMOD register.

During power-up there is a time delay associated with

the 32768Hz oscillator waiting for it to start-up. To mini-

mise this time delay, bit 4 of the RTCC register, known

as the QOSC bit, is provided to have a quick start-up

function. During a power-up condition, this bit will be

cleared to zero which will initiate the 32768Hz oscillator

quick start-up function. 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 recommended that the application pro-

gram should set the QOSC bit high for about 2 seconds

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

condition the QOSC bit is set to, the 32768Hz oscillator

will always function normally, only there is more power

consumption associated with the quick start-up function.

Note that if this external 32768Hz oscillation option is

not selected, the PC3/OSC3 and PC4/OSC4 pins are

free for use as normal I/O pins.

External Oscillator

The system clock can also be supplied by an externally

supplied clock giving users a method of synchronising

their external hardware to the microcontroller operation.

This is selected using a configuration option and supply-

ing the clock on pin OSC1. Note that if this external sys-

tem clock option is selected, as it requires OSC1

external pin for its operation, the PC2/OSC2 pin is free

for use as normal I/O pin. The internal oscillator circuit

contains a filter circuit to reduce the possibility of erratic

operation due to noise on the oscillator pin.

System Operating Modes

The devices have the ability to operate in several differ-

ent modes. This range of operating modes, known as

Normal Mode, Slow Mode, Idle Mode and Sleep Mode,

allow the devices to run using a wide range of different

slow and fast clock sources. The devices also possess

the ability to dynamically switch between different clocks

and operating modes. With this choice of operating

functions, and a HALT instruction, users are provided

with a high degree of flexibility to ensure they obtain op-

timal performance from the device, in terms of operating

speed and power requirements.

Operating Mode Clock Sources

In discussing the system clocks for the devices, they

can be seen as having a dual clock mode. These dual

clocks are what are known as a High Oscillator and the

other as a Low Oscillator. The High and Low Oscillator

are the system clock sources and can be selected dy-

namically using the HLCLK bit in the CLKMOD register.

The High Oscillator has the internal name fM whose source

is selected using a configuration option from a choice of ei-

ther an external crystal/resonator, external RC oscillator,

internal RC oscillator or external clock source.

The Low Oscillator clock source, has the internal name

fSL, whose source is also selected by configuration op-

tion from a choice of either an external 32768Hz oscilla-

tor or the internal 32K_INT oscillator. This internal fSL,fM

clock, is further modified by the SLOWC0~SLOWC2

bits in the CLKMOD register to provide the low fre-

quency clock source fSLOW.

An additional sub internal clock, with the internal name

fSUB, is a 32kHz clock source which can be sourced from

either the internal 32K_INT oscillator or an external

32768Hz crystal, selected by configuration option. To-

O S C 4

O S C 3

C 2

C 1

R 1

3 2 7 6 8 H z

External 32768Hz Oscillator

I n t e r n a l

3 2 K _ I N T

f R C 3 2 K

Internal 32K_INT Oscillator

HT45R37V Datasheet(PDF) 67 Page - Holtek Semiconductor Inc

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