Power On/Off Circuit And Power Saving Circuit; 1 ] Power On/Off Circuit - Brother PT-1800 Service Manual

3.2.4

Power ON/OFF Circuit and Power Saving Circuit

[ 1 ] Power ON/OFF circuit

Figure 3.2-5 shows a circuit diagram of the power On/Off key. The CPU processes the
On/Off key state in a sequence quite different from other keys although it is on the key
pad.
If you press the On/Off key, the signal IRQ0 interrupts the CPU on pin 31. Then, the
CPU switches pin 31 to the input port and enters the process which eliminates the
chattering signal components in the signal IRQ0.
Powering-on sequence (See Figure 3.2-6.)
1) Interrupted by IRQ0, the CPU starts the chattering elimination program module.
2) If pin 31 is Low for 10 ms, the CPU recognizes the On/Off key as being pressed
and then transfers the control to the main program module. If pin 31 is High, the
CPU interprets the input as a noise and waits for the next IRQ0.
3) Even if releasing the On/Off key makes a chattering noise so as to generate IRQ0,
the CPU interprets the signal as a noise for 10 ms (between ƒ and „) and then
transfers the control to the main program module.
Figure 3.2-5 Power ON/OFF Circuit
Figure 3.2-6 Powering-on and -off Sequence
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