HT46C64 datasheet(13/47 Pages) HOLTEK

13 / 47 page

HT46R64/HT46C64

Rev. 1.40

September 21, 2004

OSC2 with pull-high resistor, which can be used to syn-

chronize external logic. The RC oscillator provides the

most cost effective solution. However, the frequency of

the oscillation may vary with VDD, temperature, and the

chip itself due to process variations. It is therefore, not

suitable for timing sensitive operations where accurate

oscillator frequency is desired.

On the other hand, if the crystal oscillator is selected, a

crystal across OSC1 and OSC2 is needed to provide the

feedback and phase shift required for the oscillator, and

no other external components are required. A resonator

may be connected between OSC1 and OSC2 to replace

the crystal and to get a frequency reference, but two ex-

ternal capacitors in OSC1 and OSC2 are required.

There is another oscillator circuit designed for the real

time clock. In this case, only the 32.768kHz crystal oscil-

lator can be applied. The crystal should be connected

between OSC3 and OSC4.

The RTC oscillator circuit can be controlled to oscillate

quickly by setting the

²QOSC² bit (bit 4 of RTCC). It is

recommended to turn on the quick oscillating function

upon power on, and then turn it off after 2 seconds.

The WDT oscillator is a free running on-chip RC oscilla-

tor, and no external components are required. Although

the system enters the power down mode, the system

clock stops, and the WDT oscillator still works with a pe-

riod of approximately 65

ms at 5V. The WDT oscillator

can be disabled by options to conserve power.

Watchdog Timer

- WDT

The WDT clock source is implemented by a dedicated

RC oscillator (WDT oscillator) or an instruction clock

(system clock/4) or a real time clock oscillator (RTC os-

cillator). The timer is designed to prevent a software

malfunction or sequence from jumping to an unknown

location with unpredictable results. The WDT can be

disabled by options. But if the WDT is disabled, all exe-

cutions related to the WDT lead to no operation.

Once an internal WDT oscillator (RC oscillator with pe-

riod 65

ms at 5V normally) is selected, it is divided by

12~215 (by option to get the WDT time-out period). The

minimum period of WDT time-out period is about

300ms~600ms. This time-out period may vary with tem-

perature, VDD and process variations. By selection the

WDT option, longer time-out periods can be realized. If

the WDT time-out is selected 2

15, the maximum time-out

period is divided by 2

15~216about 2.1s~4.3s. If the WDT

oscillator is disabled, the WDT clock may still come from

the instruction clock and operate in the same manner

except that in the halt state the WDT may stop counting

and lose its protecting purpose. In this situation the logic

can only be restarted by external logic. If the device op-

erates in a noisy environment, using the on-chip RC os-

cillator (WDT OSC) is strongly recommended, since the

HALT will stop the system clock.

The WDT overflow under normal operation initializes a

²chip reset² and sets the status bit ²TO². In the HALT

mode, the overflow initializes a

²warm reset², and only

the PC and SP are reset to zero. To clear the contents of

the WDT, there are three methods to be adopted, i.e.,

external reset (a low level to RES), software instruction,

and a

²HALT² instruction. There are two types of soft-

ware instructions;

²CLR WDT² and the other set -²CLR

WDT1

² and ²CLR WDT2². Of these two types of instruc-

tion, only one type of instruction can be active at a time

depending on the options

-²CLR WDT² times selection

option. If the

²CLR WDT² is selected (i.e., CLR WDT

times equal one), any execution of the

²CLR WDT² in-

struction clears the WDT. In the case that

²CLR WDT1²

and

²CLR WDT2² are chosen (i.e., CLR WDT times

equal two), these two instructions have to be executed

to clear the WDT; otherwise, the WDT may reset the

chip due to time-out.

Multi-function Timer

The HT46R64/HT46C64 provides a multi-function timer

for the WDT, time base and RTC but with different

time-out periods. The multi-function timer consists of an

8-stage divider and a 7-bit prescaler, with the clock

source coming from the WDT OSC or RTC OSC or the

instruction clock (i.e., system clock divided by 4). The

multi-function timer also provides a selectable fre-

quency signal (ranges from fS/2

2 to f

S/2

8) for LCD driver

circuits, and a selectable frequency signal (ranging from

fS/2

2 to f

S/2

9) for the buzzer output by options. It is rec-

ommended to select a nearly 4kHz signal for the LCD

driver circuits to have proper display.

T i m e - o u t R e s e t

2 1 5 / f S ~ 2 1 6 / f S

2 1 4 / f S ~ 2 1 5 / f S

2 1 3 / f S ~ 2 1 4 / f S

2 1 2 / f S ~ 2 1 3 / f S

S y s t e m C l o c k / 4

D i v i d e r

W D T C l e a r

O p t i o n

W D T

O S C 1 2 k H z

R T C

O S C 3 2 7 6 8 H z

C K T

f S

W D T

P r e s c a l e r

O p t i o n

f S / 2 8

Watchdog Timer

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