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HT66FM5440
Brushless DC Motor A/D Flash MCU
HT66FM5440
Brushless DC Motor A/D Flash MCU
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, after 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 commences. 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 reset exists in the form of a Low Voltage Reset, LVR, where a full reset is implemented in
situations where the power supply voltage falls below a certain threshold. 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.
Reset Functions
There are several ways in which a microcontroller reset can occur, through events occurring
internally.
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.
V
DD
Power-on Reset
SST Time-o�t
t
RSTD
Power-on 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 and provide
s
an MCU reset should the value fall below a certain predefined level.
The LVR function is always enabled with a specific LVR voltage V
LVR
. If the supply voltage of
the device drops to within a range of 0.9V~V
LVR
such as might occur when changing the battery in
battery powered applications, 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 range between 0.9V~V
LVR
must exist for a time greater than that specified by t
LVR
in
the LVR/LVD characteristics. If the low supply voltage state does not exceed this value, the LVR
will ignore the low supply voltage and will not perform a reset function. The actual V
LVR
value is
fixed at 3.8V by setting the LVS bits in the LVRC register. If the LVS7~LVS0 bits are changed to
some different values by environmental noise, the LVR will reset the device after 2~3 f
LIRC
clock
cycles. When this happens, the LRF bit in the CTRL register will be set to 1. After power on the
register will have the value of 01010101B. Note that the LVR function will be automatically
disabled when the device enters the SLEEP/IDLE mode.