HT66F20/HT66F30/HT66F40/HT66F50/HT66F60
HT66FU30/HT66FU40/HT66FU50/HT66FU60
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.
High Speed Oscillation
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
register. The high speed system clock can be sourced
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. If fL 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.
HXT
ERC
fH
6-stage Prescaler
HIRC
High Speed Oscillation
Configuration Option
Low Speed Oscillation
fH/2
fH/4
fH/8
fH/16
fH/32
LIRC
fH/64
fL
L XT
Low Speed Oscillation
Configuration Option
fSUB
HLCLK,
CKS2~CKS0 bits
fS YS
Fast Wake-up from SLEEP Mode or
IDLE Mode Control (for HXT only)
fTB C
fSYS /4
fTB
Time Base
TBCK
fS UB
fS
fSYS/4
WDT
Configuration Option
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.
Rev. 1.10
37
February 1, 2010