HT48C05 Ver la hoja de datos (PDF) - Holtek Semiconductor

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HT48C05

Cost-Effective I/O Type 8-Bit MCU

Holtek Semiconductor 

HT48R05A-1/HT48C05/HT48R06A-1/HT48C06

It is recommended that a program does not use the

²CALL subroutine² within the interrupt subroutine. In-

terrupts often occur in an unpredictable manner or need

to be serviced immediately in some applications. If only

one stack is left and enabling the interrupt is not well

controlled, the original control sequence will be dam-

aged once the ²CALL² operates in the interrupt subrou-

tine.

Oscillator Configuration

There are two oscillator circuits in the microcontroller.

V DD

O SC1

470pF

O SC1

O SC2

fS Y S /4

O SC2

N M O S O p e n D r a in

C r y s ta l O s c illa to r

R C O s c illa to r

System Oscillator

Both are designed for system clocks, namely the RC os-

cillator and the Crystal oscillator, which are determined

by the options. No matter what oscillator type is se-

lected, the signal provides the system clock. The HALT

mode stops the system oscillator and ignores an exter-

nal signal to conserve power.

If an RC oscillator is used, an external resistor between

OSC1 and VDD is required and the resistance must

range from 24kW to 1MW. The system clock, divided by

4, is available on OSC2, which can be used to synchro-

nize external logic. The RC oscillator provides the most

cost effective solution. However, the frequency of oscil-

lation may vary with VDD, temperatures and the chip it-

self due to process variations. It is, therefore, not

suitable for timing sensitive operations where an accu-

rate oscillator frequency is desired.

If the Crystal oscillator is used, 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. Instead of a crystal, a resona-

tor can also be connected between OSC1 and OSC2 to

get a frequency reference, but two external capacitors in

OSC1 and OSC2 are required (If the oscillating fre-

quency is less than 1MHz).

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

tor, and no external components are required. Even if

the system enters the power down mode, the system

clock is stopped, but the WDT oscillator still works with a

period of approximately 65ms@5V. The WDT oscillator

can be disabled by options to conserve power.

Watchdog Timer - WDT

The clock source of WDT is implemented by a dedicated

RC oscillator (WDT oscillator) or instruction clock (sys-

tem clock divided by 4), decided by options. This timer is

designed to prevent a software malfunction or sequence

from jumping to an unknown location with unpredictable

results. The Watchdog Timer can be disabled by an op-

tion. If the Watchdog Timer is disabled, all the execu-

tions related to the WDT result in no operation.

Once the internal WDT oscillator (RC oscillator with a

period of 65ms@5V normally) is selected, it is first di-

vided by 256 (8-stage) to get the nominal time-out pe-

riod of approximately 17ms@5V. This time-out period

may vary with temperatures, VDD and process varia-

tions. By invoking the WDT prescaler, longer time-out

periods can be realized. Writing data to WS2, WS1,

WS0 (bit 2,1,0 of the WDTS) can give different time-out

periods. If WS2, WS1, and WS0 are all equal to 1, the di-

vision ratio is up to 1:128, and the maximum time-out

period is 2.1s@5V seconds. If the WDT oscillator is dis-

abled, 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 pro-

tecting purpose. In this situation the logic can only be re-

started by external logic. The high nibble and bit 3 of the

WDTS are reserved for user¢s defined flags, which can

be used to indicate some specified status.

If the device operates in a noisy environment, using the

on-chip RC oscillator (WDT OSC) is strongly recom-

mended, since the HALT will stop the system clock.

S y s te m C lo c k /4

W DT

O SC

O p tio n

S e le c t

8 - b it C o u n te r

W D T P r e s c a le r

7 - b it C o u n te r

8 -to -1 M U X

W S 0~W S 2

Watchdog Timer

W D T T im e - o u t

Rev. 1.10

10

June 9, 2004

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