
Rev. 2.50
56
��ne 22� 20��
Rev. 2.50
5�
��ne 22� 20��
HT66F20/HT66F30/HT66F40/HT66F50/HT66F60
HT66FU30/HT66FU40/HT66FU50/HT66FU60
A/D Flash MCU with EEPROM
HT66F20/HT66F30/HT66F40/HT66F50/HT66F60
HT66FU30/HT66FU40/HT66FU50/HT66FU60
A/D Flash MCU with EEPROM
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 battery powered portable applications. The fast clocks required 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 performance/power ratio.
System Clock
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 configuration options and register
programming, a clock system can be configured to obtain maximum application performance.
The main system clock, can come from either a high frequency, f
H
, or low frequency, f
L
, 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 f
L
. If f
L
is selected 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 f
H
/2~f
H
/64.
There are two additional internal clocks for the peripheral circuits, the substitute clock, f
SUB
, and
the Time Base clock, f
TBC
. Each of these internal clocks are sourced by either the LXT or LIRC
oscillators, selected via configuration options. The f
SUB
clock is used to provide a substitute clock for
the microcontroller just after a wake-up has occurred to enable faster wake-up times.
Together with f
SYS
/4 it is also used as one of the clock sources for the Watchdog timer. The fTBC
clock is used as a source for the Time Base interrupt functions and for the TMs.