A technician breadboards the keyboard entry circuit of Figure 9-16 and tests its operation by trying to enter a series of three-digit numbers. He finds that sometimes the digit 0 is entered instead of the digit he pressed. He also observes that it happens with all of the keys more or less randomly, although it is worse for some keys than others. He replaces all of the ICs, and the malfunction persists. Which of the following circuit faults would explain his observations? Explain each choice.

(a) The technician forgot to ground the unused inputs of the OR gate.

(b) He has mistakenly used Q̅ instead of Q from the one-shot.

(c) The switch bounce from the digit keys lasts longer than 20 ms.

(d) The Y and Z outputs are shorted together.

Figure 9-16

Transcribed Image Text:

Digit kays +5V 1 kn For switch bounce T OS Q = 20 ms Pull-up resistor for each input 74LS147 decimal- to-BCD encoder ->CLK CLEAR 1 k MSB ā₂ CLA +5 V LSB X J CLK CLA To CLR inputs of Qo-Q₁1 PRE CLK D D D D Ond CLK D D D D D To decoder/drivers and displays Q₁₁ D CLK 0₂ CLK 0₂ CLK CLK CLK Q₁ CLK 0₂ CLK Q₂ CLK D 0₂ CLK BCD code for MSD Second MSD LSD