HT56R666 View Datasheet(PDF) - Holtek Semiconductor
Part Name
Description
MFG CO.
'HT56R666' PDF : 104 Pages View PDF
Another reset exists in the form of a Low Voltage Reset,
LVR, where a full reset, similar to the RES reset is imple-
mented 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 ex-
ternally:
· 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 ad-
dress, 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 recom-
mended 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.
VDD
RES
S S T T im e - o u t
0 .9 V D D
tR S T D
C h ip R e s e t
Power-On Reset Timing Chart
For most applications a resistor connected between
VDD and the RES pin and a capacitor connected be-
tween VSS and the RES pin will provide a suitable ex-
ternal 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 Reset Circuit shown
is recommended.
More information regarding external reset circuits is
located in Application Note HA0075E on the Holtek
website.
HT56R66/HT56R666
V DD
0 .0 1 m F * *
VDD
1N 4148*
10kW ~
100kW
0 .1 ~ 1 m F
300W *
R E S /P A 7
VSS
Note:
²*² It is recommended that this component is
added for added ESD protection
²**² It is recommended that this component is
added in environments where power line noise
is significant
External RES Circuit
· 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 reset, the Program
Counter will reset to zero and program execution initi-
ated from this point.
RES
S S T T im e - o u t
0 .4 V D D
0 .9 V D D
tR S T D
C h ip R e s e t
RES Reset Timing Chart
· Low Voltage Reset - LVR
The microcontroller contains a low voltage reset circuit
in order to monitor the supply voltage of the device,
which is selected via a configuration option. If the supply
voltage of the device drops to within a range of
0.9V~VLVR such as might occur when changing the bat-
tery, the LVR will automatically reset the device inter-
nally. The LVR includes the following specifications: For
a valid LVR signal, a low voltage, i.e., a voltage in the
range between 0.9V~VLVR must exist for greater than the
value tLVR specified in the A.C. characteristics. If the low
voltage state does not exceed 1ms, the LVR will ignore it
and will not perform a reset function. One of a range of
specified voltage values for VLVR can be selected using
configuration options. The VLVR value will be selected as
a pair in conjunction with a Low Voltage Detect value.
LV R
tR S T D
S S T T im e - o u t
C h ip R e s e t
Low Voltage Reset Timing Chart
Rev. 1.40
74
May 11, 2012
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