HT66F016 View Datasheet(PDF) - Holtek Semiconductor
Part Name
Description
MFG CO.
'HT66F016' PDF : 116 Pages View PDF
HT66F016/HT66F017/HT68F016/HT68F017
HT66F016R/HT66F017R/HT68F016R/HT68F017R
Reset and Initialisation
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, af-
ter 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 com-
mences. 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.
Another type of reset is when the Watchdog Timer over-
flows 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 is implemented in situa-
tions where the power supply voltage falls below a cer-
tain 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.
VDD
tR S D T + tS S T
In te rn a l R e s e t
Note: tRSTD is power-on delay, typical time=50ms
Power-On Reset Timing Chart
· RES Pin
As the reset pin is shared with PA.7, the reset function
must be selected using a configuration option. Al-
though 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.
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 Enhanced Reset Cir-
cuit shown is recommended.
V DD
0 .0 1 m F * *
VDD
1N 4148*
10kW ~
100kW
0 .1 ~ 1 m F
300W *
P A 7 /R E S
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
More information regarding external reset circuits is
located in Application Note HA0075E on the Holtek
website.
Pulling the RES Pin low using external hardware will
also execute a device reset. In this case, as in the
case of other resets, the Program Counter will reset to
zero and program execution initiated from this point.
RES
0 .4 V D D
0 .9 V D D
tR S T D + tS S T
In te rn a l R e s e t
Note: tRSTD is power-on delay, typical time=100ms
RES Reset Timing Chart
· Low Voltage Reset - LVR
The microcontroller contains a low voltage reset cir-
cuit in order to monitor the supply voltage of the de-
vice. The LVR function is always enabled with a
specific LVR voltage VLVR. If the supply voltage of the
device drops to within a range of 0.9V~VLVR such as
might occur when changing the battery, the LVR will
automatically reset the device internally and the LVRF
bit in the CTRL register will also be set to 1. For a valid
LVR signal, a low supply voltage, i.e., a voltage in the
Rev. 1.40
42
November 22, 2016
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