Reset And Initialisation; Reset Functions - Holtek HT67F2350 Manual

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 already running, the
In such a case, known as a normal operation reset, some of the microcontroller registers remain
unchanged allowing the microcontroller to preceed with normal operation after the reset line is
allowed to return high.
The Watchdog Timer overflow is one of many reset types and will reset the microcontroller. Another
reset exists in the form of a Low Voltage Reset, LVR, where a full reset, similar to the
implemented in situations where the power supply voltage falls below a certain threshold. 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.

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.
Rev. 1.60
Advanced A/D Flash MCU with LCD & EEPROM
V
DD
Power-on Reset
t
RSTD
SST Time-out
Note: t is power-on delay with typical time=48 ms
RSTD
Power-On Reset Timing Chart
98
HT67F2350/HT67F2360
HT67F2370/HT67F2390
RES line is forcefully pulled low.
RES reset is
May 16, 2019