Rev. 1.10
44
November 26, 2019
Rev. 1.10
45
November 26, 2019
HT68FB240
USB Low Speed Flash MCU
Standby Current Considerations
As the main reason for entering the SLEEP or IDLE Mode is to keep the current consumption of the
device to as low a value as possible, perhaps only in the order of several micro-amps except in the
IDLE1 Mode, there are other considerations which must also be taken into account by the circuit
designer if the power consumption is to be minimised. Special attention must be made to the I/O pins
on the device. All high-impedance input pins must be connected to either a fixed high or low level as
any floating input pins could create internal oscillations and result in increased current consumption.
This also applies to the device which has different package types, as there may be unbonbed pins.
These must either be setup as outputs or if setup as inputs must have pull-high resistors connected.
Care must also be taken with the loads, which are connected to I/O pins, which are setup as outputs.
These should be placed in a condition in which minimum current is drawn or connected only to
external circuits that do not draw current, such as other CMOS inputs. Also note that additional
standby current will also be required if the LIRC oscillator is
enabled.
In the IDLE1 Mode the system oscillator is on, if the system oscillator is from the high speed system
oscillator, the additional standby current will also be perhaps in the order of several hundred micro-
amps.
Wake-up
After the system enters the SLEEP or IDLE Mode, it can be woken up from one of various sources
listed as follows:
• An
external
or USB
reset
•
An external falling edge on Ports
•
A system interrupt
•
A WDT overflow
If the system is woken up by an external or USB reset, the device will experience a full system reset,
however, if the device is woken up by a WDT overflow, a Watchdog Timer reset will be initiated.
Although 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
system power-up or executing the clear Watchdog Timer instructions and is set when executing the
"HALT" instruction. The TO flag is set if a WDT time-out occurs, and causes a wake-up that only
resets the Program Counter and Stack Pointer, the other flags remain in their original status.
Each pin on Ports can be setup using the PAWU and PXWU registers to permit a negative transition
on the pin to wake-up the system. When a Port pin wake-up occurs, the program will resume
execution at the instruction following the "HALT" instruction. 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 program will resume execution at
the instruction following the "HALT" 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 situation is where the related interrupt
is enabled and the stack is not full, in which case the regular interrupt response takes place. If an
interrupt request flag is set high before entering the SLEEP or IDLE Mode, the wake-up function of
the related interrupt will be disabled.