HT46R064D/065D/066D
Enhanced A/D Type 8-Bit OTP MCU with LED Driver
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.
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.
Note: tRSTD is power-on delay, typical time=50ms
Power-On Reset Timing Chart
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. 1.00
35
January 12, 2011
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
