HT45R37
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RES Pin Reset
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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. The RES pin is shared with an I/O
pin so its function must be first selected using a config-
uration option.
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WDT Time-out Reset during Power Down
Reset Initial Conditions
The different types of reset described affect the reset
flags in different ways. These flags, known as PDF and
TO are located in the status register and are controlled
by various microcontroller operations, such as the
Power Down function or Watchdog Timer. The reset
flags are shown in the table:
0
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TO PDF
RESET Conditions
RES Reset Timing Chart
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1
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RES reset during power-on
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Low Voltage Reset - LVR
RES or LVR reset during normal operation
WDT time-out reset during normal operation
WDT time-out reset during Power Down
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.
Note: ²u² stands for unchanged
The following table indicates the way in which the vari-
ous components of the microcontroller are affected after
a power-on reset occurs.
Item
Program Counter
Interrupts
Condition After RESET
Reset to zero
All interrupts will be disabled
Clear after reset, WDT begins
counting
WDT
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Timer/Event Counter Timer Counter will be turned off
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The Timer Counter Prescaler
Prescaler
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will be cleared
Low Voltage Reset Timing Chart
Input/Output Ports
Stack Pointer
I/O ports will be setup as inputs
Stack Pointer will point to the
top of the stack
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Watchdog Time-out Reset during Normal Operation
The Watchdog time-out Reset during normal opera-
tion is the same as a hardware RES pin reset except
that the Watchdog time-out flag TO will be set to ²1².
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WDT Time-out Reset during Normal Operation
Timing Chart
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Watchdog Time-out Reset during Power Down
The Watchdog time-out Reset during Power Down is
a little different from other kinds of reset. Most of the
conditions remain unchanged except that the Pro-
gram Counter and the Stack Pointer will be cleared to
²0² and the TO flag will be set to ²1². Refer to the A.C.
Characteristics for tSST details.
Rev. 1.20
59
February 25, 2011
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
