Key board circuit design

As I just mentioned, we must make a choice to select each of keyboard circuit. The keyboard must stable in any conditions. There will be 2 ways to implement the keyboard. One of it is the analog to digital keyboard , the other one is the build in keyboard scanner.

A/D keyboard

       The output of A/D keyboard is analog one, so it must suffer from noise in circuit, so the largest output interval among keys get the best noise immunity. So if n keys used and power by VCC, the max interval will be VCC/n. we can have 4 configurations of keyboard like figure 1. The direct way is use n same resistors to divide the VCC like figure 1(a), when the nth key is pressed the output is n/VCC, to give a no key pressed status, a pull-up resistor should used to pull the output. This pull-up resistor should much great than the voltage-divide resistors. This circuit is suffered from key parasitic resistor in humidity condition, because of the high input impedance of A/D converter. Figure 1(a) can be changed to figure 1(b), just remove R3, and change pull-up resistors to a right one. Though this circuit is not sensitive to parasitic resistor, but it is sensitive to noise, because the output interval is smaller. We can change the resistors of R0 R1 R2 to appropriate one to enlarge the output interval. Circuit like figure 1(c) has a problem of accumulating resistor value deviation. Use circuit like figure 1(d) seem better.

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