HT45F43 datasheet(49/152 Pages) HOLTEK

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Programming Considerations

The HXT and LXT oscillators both use the same SST counter. For example, if the system is woken up

from the SLEEP0 Mode and both the HXT and LXT oscillators need to start-up from an off state. The

LXT oscillator uses the SST counter after HXT oscillator has finished its SST period.

· If the device is woken up from the SLEEP0 Mode to the NORMAL Mode, the high speed system

oscillator needs an SST period. The device will execute first instruction after HTO is

²1². At this

time, the LXT oscillator may not be stability if fSUB is from LXT oscillator. The same situation occurs

in the power-on state. The LXT oscillator is not ready yet when the first instruction is executed.

· If the device is woken up from the SLEEP1 Mode to NORMAL Mode, and the system clock source

is from HXT oscillator and FSTEN is

²1², the system clock can be switched to the LXT or LIRC

oscillator after wake up.

· There are peripheral functions, such as WDT, TMs and SIM, for which the fSYS is used. If the system

clock source is switched from fH to fL, the clock source to the peripheral functions mentioned above

will change accordingly.

· The on/off condition of fSUB and fS depends upon whether the WDT is enabled or disabled as the

WDT clock source is selected from fSUB.

Watchdog Timer

The Watchdog Timer is provided to prevent program malfunctions or sequences from jumping to

unknown locations, due to certain uncontrollable external events such as electrical noise.

Watchdog Timer Clock Source

The Watchdog Timer clock source is provided by the internal clock, fS, which is in turn supplied by one

of two sources selected by configuration option: fSUB or fSYS/4. The fSUB clock can be sourced from either

the LXT or LIRC oscillators, again chosen via a configuration option. The Watchdog Timer source

clock is then subdivided by a ratio of 2

13 to 220 to give longer timeouts, the actual value being chosen

using the WS2~WS0 bits in the WDTC register. The LIRC internal oscillator has an approximate

period of 32kHz at a supply voltage of 5V.

However, it should be noted that this specified internal clock period can vary with VDD, temperature

and process variations. The LXT oscillator is supplied by an external 32.768kHz crystal. The other

Watchdog Timer clock source option is the fSYS/4 clock. The Watchdog Timer clock source can

originate from its own internal LIRC oscillator, the LXT oscillator or fSYS/4. It is divided by a value of

13 to 220, using the WS2~WS0 bits in the WDTC register to obtain the required Watchdog Timer

time-out period.

HT45F23/HT45F43

8-Bit Flash MCU with Op Amps & Comparators

Rev. 1.20

September 15, 2011

HT45F43 Datasheet(PDF) 49 Page - Holtek Semiconductor Inc