HT66FB540 Datasheet, PDF (73/245 Pages) Holtek Semiconductor Inc – A/D Flash USB 8-Bit MCU with SPI

HT66FB540/HT66FB550/HT66FB560
A/D Flash USB 8-Bit MCU with SPI
Reset and Initialisation
A reset function is a fundamental part of any microcontroller ensuring that these devices can be set
to some predetermined condition irrespective of outside parameters. A hardware reset will of course
be automatically implemented after these devices are powered-on, however there are a number of
other hardware and software reset sources that can be implemented dynamically when these devices
are running.
Reset Overview
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 instructions commence execution.
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.
These devices provide several reset sources to generate the internal reset signal, providing extended
MCU protection. The different types of resets are listed in the accompanying table.
No.
Reset Name
Abbreviation Indication Bit Register
Notes
1 Power-On Reset
POR
—
— Auto generated at power on
2 Reset Pin
RES
—
— Hardware Reset
3 Low-Voltage Reset
4 Watchdog Reset
LVR
WDT
LRF
CTRL Low VDD voltage
TO
STATUS
—
5
WDTC Register Setting
Software Reset
—
WRF
CTRL Write to WDTC register
Reset Source Summary
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 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.
Rev. 1.00
73
April 03, 2013