Rev. 1.50
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HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
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.
In addition to the power-on reset, 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 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.
V
DD
Power-on Reset
SST Time-o�t
t
RSTD
Note: t
RSTD
is power-on delay with typical time = 50 ms
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. 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, 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 LVD/LVR
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 can be selected by
the LVS bits in the LVRC register. If the LVS7~LVS0 bits have any other value, which may perhaps
occur due to adverse environmental conditions such as 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 power down mode.