HT48CA0 Просмотр технического описания (PDF) - Holtek Semiconductor

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HT48CA0

8-bit Microcontroller

Holtek Semiconductor 

HT48CA0

Oscillator configuration

There are two oscillator circuits in the HT48CA0.

System oscillator

Both are designed for system clocks; the RC

oscillator and the Crystal oscillator, which are

determined by mask options. No matter what

oscillator type is selected, the signal provides

the system clock. The HALT mode stops the

system oscillator and ignores the external sig-

nal to conserve power.

If an RC oscillator is used, an external resistor

between OSC1 and VSS in needed and the re-

sistance must range from 51kΩ to 1MΩ. The

system clock, divided by 4, is available on

OSC2, which can be used to synchronize exter-

nal logic. The RC oscillator provides the most

cost effective solution. However, the frequency

of the oscillation may vary with VDD, tempera-

ture and the chip itself due to process vari-

ations. It is, therefore, not suitable for timing

sensitive operations where accurate oscillator

frequency is desired.

If the Crystal oscillator is used, a crystal across

OSC1 and OSC2 is needed to provide the feed-

back and phase shift for the oscillator. No other

external components are needed. Instead of a

crystal, the resonator can also be connected

between OSC1 and OSC2 to get a frequency

reference, but two external capacitors in OSC1

and OSC2 are required.

Watchdog timer – WDT

The clock source of the WDT is implemented by

instruction clock (system clock divided by 4).

The clock source is processed by a frequency

divider and a prescaller to yield various time

out periods.

WDT time out period =

Clock Source

2n

Where n= 8~11 selected by mask option.

This timer is designed to prevent a software

malfunction or sequence jumping to an un-

known location with unpredictable results. The

Watchdog Timer can be disabled by mask op-

tion. If the Watchdog Timer is disabled, all the

executions related to the WDT result in no op-

Watchdog timer

11

23rd July ’98

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