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.
®
RES Pin Reset
This type of reset occurs when the microcontroller is already running and the RES
pin is forcefully
pulled low by external hardware such as an external switch. In this case as in the case of other re
-
set, the Program Counter will reset to zero and program execution initiated from this point.
66
A/D Type MCU
R E S
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
0 . 4 V
D D
t
R S T D
RES Reset Timing Chart
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
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