Interrupt scan method matrix keyboard circuit

Shown is the hardware schematic diagram of the interrupt scanning method. All column line outputs are low level . When no key is pressed, all row
lines are high level. After passing through the 4-input NAND gate, the output is low level, and then sent to the interrupt application section IRQ2. This No interruption will occur. When any
key is pressed, the row line of the corresponding key becomes low level, causing the output of the 4-input NAND gate to become high level . At this time, IRQ2 generates a positive transition and applies for an interrupt
to the CPU. . After the CPU responds to the interrupt, it calls the keyboard program 1 of the aforementioned timing scan method to find the pressed key and perform
corresponding processing.