HT68F20 datasheet(36/210 Pages) HOLTEK

36 / 210 page

HT68F20/HT68F30/HT68F40/HT68F50/HT68F60

Rev. 1.00

November 3, 2009

Operating Modes and System Clocks

Present day applications require that their

microcontrollers have high performance but often still

demand that they consume as little power as possible,

conflicting requirements that are especially true in bat-

tery powered portable applications. The fast clocks re-

quired for high performance will by their nature increase

current consumption and of course vice-versa, lower

speed clocks reduce current consumption. As Holtek

has provided these devices with both high and low

speed clock sources and the means to switch between

them dynamically, the user can optimise the operation of

their microcontroller to achieve the best perfor-

mance/power ratio.

System Clocks

The device has many different clock sources for both

the CPU and peripheral function operation. By providing

the user with a wide range of clock options using config-

uration options and register programming, a clock sys-

tem can be configured to obtain maximum application

performance.

The main system clock, can come from either a high fre-

quency, fH, or low frequency, fL, source, and is selected

using the HLCLK bit and CKS2~CKS0 bits in the SMOD

from either an HXT, ERC or HIRC oscillator, selected via

a configuration option. The low speed system clock

source can be sourced from internal clock fL.IffL is se-

lected then it can be sourced by either the LXT or LIRC

oscillators, selected via a configuration option. The

other choice, which is a divided version of the high

speed system oscillator has a range of fH/2~fH/64.

There are two additional internal clocks for the periph-

eral circuits, the substitute clock, fSUB, and the Time

Base clock, fTBC. Each of these internal clocks are

sourced by either the LXT or LIRC oscillators, selected

via configuration options. The fSUB clock is used to pro-

vide a substitute clock for the microcontroller just after a

wake-up has occurred to enable faster wake-up times.

fTBC

fSYS/4

TBCK

fSUB

fSYS/4

fTB

Configuration Option

Time Base

WDT

HXT

ERC

HIRC

LIRC

LXT

6-stage Prescaler

High Speed Oscillation

Low Speed Oscillation

Configuration Option

High Speed Oscillation

Configuration Option

fH/2

fH/4

fH/8

fH/16

fH/32

fH/64

HLCLK,

CKS2~CKS0 bits

Fast Wake-up from SLEEP Mode or

IDLE Mode Control (for HXT only)

fSYS

fSUB

System Clock Configurations

Note:

When the system clock source fSYS is switched to fL from fH, the high speed oscillation will stop to conserve the

power. Thus there is no fH~fH/64 for peripheral circuit to use.

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