Rev. 1.21
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HT66F488/HT66F489
A/D Flash MCU with EEPROM
HT66F488/HT66F489
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.
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. 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.
Reset Functions
There are several 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.
Note: t
RSTD
is power-on delay, typical time=50ms
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, which is always enabled selected by the configuration option. 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 to1.
F
or a valid LVR signal, a low voltage, i.e., a voltage in the range between 0.9V~ V
LVR
must exist for greater than the value t
LVR
specified in the A.C. characteristics. If the low 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
can be selected by the LVS7~LVS0 bits in the LVRC register. If the
LVS7~LVS0 bits are changed to some certain values by the environmental noise, the LVR will reset
the device after 2~3 LIRC/LXT 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.
Note:t
RSTD
is power-on delay, typical time=50ms
Low Voltage Reset Timing Chart