HT66F13 datasheet(44/173 Pages) HOLTEK

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Resetand Initialisation

A reset function is a fundamental part of any microcontroller ensuring that the device can be set to

some predetermined condition irrespective of outside parameters. The most important reset condition

is after power is first applied to the microcontroller. In this case, internal circuitry will ensure that the

microcontroller, after a short delay, will be in a well defined state and ready to execute the first

program instruction. After this power-on reset, certain important internal registers will be set to

defined states before the program commences. One of these registers is the Program Counter, which

will be reset to zero forcing the microcontroller to begin program execution from the lowest Program

Memory address.

In addition to the power-on reset, situations may arise where it is necessary to forcefully apply a reset

condition when the microcontroller is running. One example of this is where after power has been

applied and the microcontroller is already running, the RES line is forcefully pulled low. In such a

case, known as a normal operation reset, some of the microcontroller registers remain unchanged

allowing the microcontroller to precede with normal operation after the reset line is allowed to return

high.

Another type of reset is when the Watchdog Timer overflows and resets the microcontroller. All types

of reset operations result in different register conditions being setup. Another reset exists in the form of

a Low Voltage Reset, LVR, where a full reset, similar to the RES reset is implemented in situations

where the power supply voltage falls below a certain threshold.

ResetFunctions

There are five ways in which a microcontroller reset can occur, through events occurring both

internally and externally:

Power-onReset

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. All the I/O port and port control registers will power up in a high condition ensuring that all

pins will be first set to inputs.

HT66F13/HT66F14/HT66F15

EnhancedA/DFlashType MCU

Rev.1.10

February9,2011

R E S

V D D

I n t e r n a l R e s e t

0 . 9 V D D

t R S T D + t S S T

Note: t

RSTD ispower-ondelay,typicaltime=100ms

Power-OnResetTimingChart

HT66F13 датащи(PDF) 44 Page - Holtek Semiconductor Inc