HT82B40A HOLTEK [Holtek Semiconductor Inc], HT82B40A Datasheet - Page 23

• Manufacturer Part Number: HT82B40A
• Description: I/O MCU with USB Interface
• Manufacturer: HOLTEK [Holtek Semiconductor Inc]

Programming Considerations
By disabling the interrupt enable bits, a requested inter-
rupt can be prevented from being serviced, however,
once an interrupt request flag is set, it will remain in this
condition in the interrupt control register until the corre-
sponding interrupt is serviced or until the request flag is
cleared by a software instruction.
It is recommended that programs do not use the CALL
subroutine instruction within the interrupt subroutine.
Interrupts often occur in an unpredictable manner or
need to be serviced immediately in some applications. If
only one stack is left and the interrupt is not well con-
trolled, the original control sequence will be damaged
once a CALL subroutine is executed in the interrupt
subroutine.
All of these interrupts have the capability of waking up
the processor when in the Power Down Mode.
Only the Program Counter is pushed onto the stack. If
the contents of the accumulator or status register are al-
tered by the interrupt service program, which may cor-
rupt the desired control sequence, then the contents
should be saved in advance.
USB Interrupt
The USB interrupts are triggered by the following USB
events causing the related interrupt request flag, USBF,
to be set.
When the interrupt is enabled, the stack is not full and
the USB interrupt is active, a subroutine call to location
04H will occur. The interrupt request flag, USBF, and the
EMI bit will be cleared to disable other interrupts.
When the PC Host accesses the FIFO of the device, the
corresponding request bit, USR, is set, and a USB inter-
rupt is triggered. So the user can easy determine which
FIFO has been accessed. When the interrupt has been
served, the corresponding bit should be cleared by firm-
ware. When the device receive a USB Suspend signal
from Host PC, the suspend line (bit0 of USC) is set and a
USB interrupt is also triggered.
Also when device receive a Resume signal from Host
PC, the resume line (bit3 of USC) is set and a USB inter-
rupt is triggered.
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
Rev. 1.10
Access of the corresponding USB FIFO from PC
A USB suspend signal from the PC
A USB resume signal from the PC
A USB Reset signal
23
power is first applied to the microcontroller. In this case,
internal circuitry will ensure that the microcontroller, af-
ter 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 com-
mences. 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, 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 force-
fully pulled low. In such a case, known as a normal oper-
ation reset, some of the microcontroller 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 re-
set operations result in different register conditions be-
ing setup.
Another reset exists in the form of a Low Voltage Reset,
LVR, where a full reset, similar to the RES reset is imple-
mented in situations where the power supply voltage
falls below a certain threshold.
Reset Functions
There are five ways in which a microcontroller reset can
occur, through events occurring both internally and ex-
ternally:
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 ad-
dress, 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.
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 a
proper reset operation. In such cases it is recom-
mended 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
an extra delay time after which the microcontroller will
begin normal operation. The abbreviation SST in the
figures stands for System Start-up Timer.
HT82B40R/HT82B40A
RSTD
is invoked to provide
September 4, 2009