HT45F23 datasheet(52/152 Pages) HOLTEK

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Reset and 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 proceed 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.

Reset Functions

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

internally and externally:

· 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. 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.

· RES Pin

As the reset pin is shared with PB.6, the reset function must be selected using a configuration option.

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 RES pin, whose additional time delay will ensure that the RES 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 RES line reaches a certain

voltage value, the reset delay time tRSTD 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.

For most applications a resistor connected between VDD and the RES pin and a capacitor connected

between VSS and the RES pin will provide a suitable external reset circuit. Any wiring connected to

the RES pin should be kept as short as possible to minimise any stray noise interference.

Rev. 1.20

September 15, 2011

HT45F23/HT45F43

8-Bit Flash MCU with Op Amps & Comparators

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: tRSTD is power-on delay, typical time=100ms

Power-On Reset Timing Chart

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