Rev. 1.10
104
November 26, 2019
Rev. 1.10
105
November 26, 2019
HT68FB240
USB Low Speed Flash MCU
TM Interrupts
The Compact Type TMs have two interrupts each. All of the TM interrupts are contained within the Multi-
function Interrupts. For each of the Compact Type TMs there are two interrupt request flags TnPF and
TnAF and two enable bits TnPE and TnAE. A TM interrupt request will take place when any of the TM
request flags are set, a situation which occurs when a TM comparator P or A match situation happens.
To allow the program to branch to its respective interrupt vector address, the global interrupt enable
bit, EMI, respective TM Interrupt enable bit, and relevant Multi-function Interrupt enable bit, MFnE,
must first be set. When the interrupt is enabled, the stack is not full and a TM comparator match
situation occurs, a subroutine call to the relevant Multi-function Interrupt vector locations, will take
place. When the TM interrupt is serviced, the EMI bit will be automatically cleared to disable other
interrupts, however only the related MFnF flag will be automatically cleared. As the TM interrupt
request flags will not be automatically cleared, they have to be cleared by the application program.
Interrupt Wake-up Function
Each of the interrupt functions has the capability of waking up the microcontroller when in the SLEEP
or IDLE Mode. A wake-up is generated when an interrupt request flag changes from low to high and
is independent of whether the interrupt is enabled or not. Therefore, even though the device is in the
SLEEP or IDLE Mode and its system oscillator stopped, situations such as external edge transitions
on the external interrupt pins, a low power supply voltage or comparator input change may cause their
respective interrupt flag to be set high and consequently generate an interrupt. Care must therefore
be taken if spurious wake-up situations are to be avoided. If an interrupt wake-up function is to be
disabled then the corresponding interrupt request flag should be set high before the device enters the
SLEEP or IDLE Mode. The interrupt enable bits have no effect on the interrupt wake-up function.
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 "CALL" 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.
