LCD Type MCU
After the system enters the Power Down Mode, it can be woken up from one of various sources
listed as follows:
An external reset
An external falling edge on Port A
A system interrupt
A WDT overflow
An external falling edge on the UART RX pin
If the system is woken up by an external reset, the device will experience a full system reset, how-
ever, if the device is woken up by a WDT overflow, a Watchdog Timer reset will be initiated. Al-
though both of these wake-up methods will initiate a reset operation, the actual source of the
wake-up can be determined by examining the TO and PDF flags. The PDF flag is cleared by a sys-
tem power-up or executing the clear Watchdog Timer instructions and is set when executing the
instruction. The TO flag is set if a WDT time-out occurs, and causes a wake-up that only re-
sets the Program Counter and Stack Pointer, the other flags remain in their original status.
If the system is woken up due to a low going edge on any wake-up configured Port A input lines, or
additionally in the case of the HT49RU80/HT49CU80 devices, a low going edge on the UART RX
pin, then the program will continue execution from the statement following the
The contents of the Data Memory and all register values will remain unchanged from the value at the
time of the
instruction execution and the program will continue running normally. Note that
each line on Port A can be independently selected to wake-up the device by a configuration option.
For the UART RX pin wake-up to be operational, the UART receiver and its corresponding wake-up
function must be enabled by setting the appropriate bits in the UART control registers.
If the system is woken up by an interrupt, then two possible situations may occur. The first is where
the related interrupt is disabled or the interrupt is enabled but the stack is full, in which case the pro-
gram will resume execution at the instruction following the
instruction. In this situation, the
interrupt which woke-up the device will not be immediately serviced, but will rather be serviced
later when the related interrupt is finally enabled or when a stack level becomes free. The other sit-
uation is where the related interrupt is enabled and the stack is not full, in which case the regular in-
terrupt response takes place. If an interrupt request flag is set to 1 before entering the Power
Down Mode, the wake-up function of the related interrupt will be disabled.
No matter what the source of the wake-up event is, once a wake-up situation occurs, a time period
equal to 1024 system clock periods will be required before normal system operation resumes.
However, if the wake-up has originated due to an interrupt, the actual interrupt subroutine execu-
tion will be delayed by an additional one or more cycles. If the wake-up results in the execution of
the next instruction following the
instruction, this will be executed immediately after the
1024 system clock period delay has ended.