Rev. 1.10
48
November 26, 2019
Rev. 1.10
49
November 26, 2019
HT68FB240
USB Low Speed Flash MCU
In addition to the power-on reset, situations may arise where it is necessary to forcefully apply a
reset condition when the microcontroller is running. One example of this is where after power has
been applied and the microcontroller is already running, the RES
line is forcefully pulled low. In
such a case, known as a normal operation reset, some of the registers remain unchanged allowing
the microcontroller to proceed with normal operation after the reset line is allowed to return high.
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
t
he form of a Low Voltage Reset, LVR, where a full reset, similar to the RES
reset is implemented in
situations where the power supply voltage falls below a certain threshold.
Reset Functions
There are several
ways in which a 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 I/O ports will be first set to inputs.
Power-On Reset Timing Chart
RES Pin
Although 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 recommended 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 t
RSTD
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
between VSS and the RES pin will provide a suitable external 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
Circuit shown is recommended.