Brother P-touch BB4, Service Manual

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3.2.11 Dial Position Detection Circuit
Dial LED scanning
Figure 3.2-15 shows the LED scan circuit that detects the dial position.
The CPU scans five LEDs. Ports LED0 through LED4 on the CPU issue a group of LED
scanning pulses. Port AN2 through AN6 act as input ports that receive LED status.
Scanning timing
The CPU turns ports LED0, LED1, LED2, LED3, and LED4 Low in this order. When not
Low, those ports are High or in high impedance. When a particular port out of them is
Low, the CPU reads the status of the corresponding port.
The CPU scans those LEDs every 50 ms. If the CPU reads the same pattern on input
ports five successive times, then it interprets it as the current dial position.
Figure 3.2-16 shows the waveforms of dial LED scanning pulse outputs.
Figure 3.2-15 Dial Position Detection Circuit