BS85B12-3 Datasheet, PDF (146/182 Pages) Holtek Semiconductor Inc – Touch Key Flash Type 8-Bit MCU with LCD/LED Driver

BS85B12-3/BS85C20-3
Touch Key Flash MCU with LCD/LED Driver
Programming Considerations
By disabling the relevant interrupt enable bits, a requested interrupt can be prevented from being
serviced, however, once an interrupt request flag is set, it will remain in this condition in the interrupt
register until the corresponding interrupt is serviced or until the request flag is cleared by the
application program.
Where a certain interrupt is contained within a Multi-function interrupt, then when the interrupt
service routine is executed, as only the Multi-function interrupt request flags, MFnF, will be
automatically cleared, the individual request flag for the function needs to be cleared by the application
program.
It is recommended that programs do not use the ²CALL² instruction within the interrupt service
subroutine. Interrupts often occur in an unpredictable manner or need to be serviced immediately. If
only one stack is left and the interrupt is not well controlled, the original control sequence will be
damaged once a CALL subroutine is executed in the interrupt subroutine.
Every interrupt has the capability of waking up the microcontroller when it is in SLEEP or IDLE
Mode, the wake up being generated when the interrupt request flag changes from low to high. If it is
required to prevent a certain interrupt from waking up the microcontroller then its respective request
flag should be first set high before enter SLEEP or IDLE Mode.
As only the Program Counter is pushed onto the stack, then when the interrupt is serviced, if the
contents of the accumulator, status register or other registers are altered by the interrupt service
program, their contents should be saved to the memory at the beginning of the interrupt service
routine.
To return from an interrupt subroutine, either a RET or RETI instruction may be executed. The RETI
instruction in addition to executing a return to the main program also automatically sets the EMI bit
high to allow further interrupts. The RET instruction however only executes a return to the main
program leaving the EMI bit in its present zero state and therefore disabling the execution of further
interrupts.
Low Voltage Detector - LVD
Each device has a Low Voltage Detector function, also known as LVD. This enabled the device to
monitor the power supply voltage, V_DD, and provide a warning signal should it fall below a certain
level. This function may be especially useful in battery applications where the supply voltage will
gradually reduce as the battery ages, as it allows an early warning battery low signal to be generated.
The Low Voltage Detector also has the capability of generating an interrupt signal.
LVD Register
The Low Voltage Detector function is controlled using a single register with the name LVDC. Three
bits in this register, VLVD2~VLVD0, are used to select one of eight fixed voltages below which a low
voltage condition will be detemined. A low voltage condition is indicated when the LVDO bit is set. If
the LVDO bit is low, this indicates that the V_DD voltage is above the preset low voltage value. The
LVDEN bit is used to control the overall on/off function of the low voltage detector. Setting the bit high
will enable the low voltage detector. Clearing the bit to zero will switch off the internal low voltage
detector circuits. As the low voltage detector will consume a certain amount of power, it may be
desirable to switch off the circuit when not in use, an important consideration in power sensitive
battery powered applications.
Rev. 1.00
140
February 1, 2011