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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 condi-
tion 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 re-
set 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 un-
changed allowing the microcontroller to proceed with normal operation after the reset line is al-
lowed 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
There are five ways in which a microcontroller reset can occur, through events occurring both inter-
nally 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, due to unstable power-on condi-
tions, an external RC network connected to the RES pin is generally recommended. This time de-
lay created by the RC network ensures that the RES pin remains low for an extended period while
the power supply stabilizes. During this time, normal operation of the microcontroller is inhibited.
After the RES line reaches a certain voltage value, the reset delay time t
RSTD
is invoked to provide
an extra delay time after which the microcontroller can begin normal operation. The abbreviation
SST in the figures stands for System Start-up Timer.
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Cost-Effective I/O Type MCU
R E S
V D D
S S T T i m e - o u t
I n t e r n a l R e s e t
0 . 9 V
D D
t
R S T D
Power-on Reset Timing Chart