As shown in the figure, the integration circuit in the lower left corner uses the characteristic that the capacitor voltage will not suddenly change to eliminate the key jitter. The upper resistance network is divided into five nodes: A, B, C and D, with red corresponding to KEY0 and green corresponding to KEY 1.

Without a key, KEY0 = KEY 1 = Va =? 1.95 volts.

When the menu is pressed, the red Vb is grounded, with KEY0 = 0.874V and key1=1.95v. ..

When DOWN is pressed, the red Vd is grounded, with KEY0 = 1.7 13V and key1=1.95v. ..

EN is pressed, green Vd is grounded, KEY0 = 1.95V, key1=1.713v. ..

It can be seen that the keyboard coding mode is expressed by different output voltages, and * * * uses four voltage levels and two output lines.