Reset And Initialization; Reset F�Nctions - Holtek HT46R003B Manual

Reset and Initialization

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 deal 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 set.
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 Reset
Although the microcontroller has an internal RC reset function, if the VDD power supply rise time
is not fast enough or does not stabilize 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 stabilize. 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 t
microcontroller will begin normal operation. The abbreviation SST in the figures stands for System
Start-up Timer.
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