HT46R068B/HT46R069B
Enhanced A/D Type 8-bit OTP MCU
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.
V
D
D
0
9
.
V
D
D
R
S
E
t
t
R
R
T
T
S
S
D
D
+
+
t
t
S
S
S
S
T
T
I
t
n
r
e
n
a
R
e
l
e
s
t
Power-On Reset Timing Chart
Note: tRSTD is power-on delay, typical time=100ms
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.
For applications that operate within an environment where more noise is present the Enhanced
Reset Circuit shown is recommended.
V
D
D
0
0
.
1
F
*
*
V
D
D
1
4
N
1
4
8
*
1
0
W
~
k
1
0
W
0
k
R
S
E
P
/
7
A
3
0
W
*
0
0
1
.
1
~
F
V
S
S
External RES Circuit
Note: "*" It is recommended that this component is added ESD protection.
"**" It is recommended that this component is added in environments where power line noise is
significant.
More information regarding external reset circuits is located in Application Note HA0075E on the
Holtek website.
Rev. 1.00
37
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HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
