HT68F14 HOLTEK [Holtek Semiconductor Inc], HT68F14 Datasheet - Page 42

• Manufacturer Part Number: HT68F14
• Description: Enhanced I/O Flash Type MCU
• Manufacturer: HOLTEK [Holtek Semiconductor Inc]

Reset and Initialisation
Rev. 1.10
Reset Functions
Power-on Reset
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 precede 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.
There are five ways in which a microcontroller reset can occur, through events occurring both
internally and externally:
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.
Note: t
RSTD
is power-on delay, typical time=100ms
Power-On Reset Timing Chart
42
Enhanced I/O Flash Type MCU
HT68F13/HT68F14/HT68F15
February 9, 2011