HT85F2270 datasheet(78/226 Pages) HOLTEK

78 / 226 page

Rev. 1.00

78 of ��6

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Standard 8051 8-Bit Flash MCU

HT85F2260/HT85F2270/HT85F2280

Power-on Reset

The most fundamental and unavoidable reset is the one that occurs after power is first applied to

the microcontroller. As well as ensuring that the Program Memory begins execution from the first

memory address, a power-on reset also ensures that certain other registers are preset to known

conditions. The entire I/O data and port mode registers will power up to ensure that all pins will be

first set to the quasi-bidirection structure.

Although the microcontroller has an internal RC reset function, if the VDD power supply rise

time is not fast enough or does not stabilise quickly at power-on, the internal reset function may be

incapable of providing proper reset operation. For this reason it is recommended that an external

RC network is connected to the RESET pin, whose additional time delay will ensure that the

RESET pin remains low for an extended period to allow the power supply to stabilise. During this

time delay, normal operation of the microcontroller will be inhibited. After the RESET line reaches

a certain voltage value, the reset delay time of tSST, which is equal to 1024 system clock pulses, is

invoked to provide an extra delay time after which the microcontroller will begin normal operation.

The abbreviation SST in the figures stands for System Start-up Timer. When the Power-on reset

takes place, the PORF bit in the RSTSRC register will be set high to indicate this reset.

VDD

SST Time-out

Chip Reset

tSST

Power-On Reset Timing

HT85F2270 Datasheet(PDF) 78 Page - Holtek Semiconductor Inc