ht82j31a Holtek Semiconductor Inc., ht82j31a Datasheet - Page 21

• Manufacturer Part Number: ht82j31a
• Description: 16 Channel A/d Mcu With Spi Interface
• Manufacturer: Holtek Semiconductor Inc.

and the EMI bit will be automatically cleared to disable
other interrupts.
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.
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, 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.
Rev. 1.00
21
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 incapa-
ble of providing a proper reset operation. In such
cases 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
which the microcontroller will begin normal operation.
The abbreviation SST in the figures stands for System
Start-up Timer.
For most circuits a resistor connected between VSS
and the RES pin and a capacitor connected between
VSS and the RES pin will suffice, however for more re-
liable operation the following circuit is recommended.
Note that as the external reset pin is also pin-shared
with PA7, if it is to be used as a reset pin, the correct
reset configuration option must be selected. If the
configuration option selects this pin to be an I/O pin,
RSTD
is invoked to provide an extra delay time after
Power-On Reset Timing Chart
Reset Circuit
September 19, 2007
HT82J31A