Question

Modify the hours stage of figure 10-18 to keep military time (00-23 hours)

SECTION 10-4/DIGITAL CLOCK PROJECT 763 AMPM tens hrs PM CLRN 74160 units hrs O] QB QC ENT QD ENP RCO units hrs 2] units-hrs[3] CLRN Tens of hours PRN Units of hours CLRN FIGURE 10-18 Detailed circuitry for the HOURS section to count tens of hours. The BCD counter is a 74160, which has two active- HIGH inputs, ENT and ENP, that are ANDed together internally to enable the count. The ENT input also enables the active-HIGH ripple carry out (RCO) that detects the BCD terminal count of 9. The ENT input and RCO output can therefore be used for synchronous counter cascading. The ENP input is tied HIGH so that the counter will increment whenever ENT is HIGH The hours counter is enabled by the minutes and seconds stages for only one clock pulse every hour. When this condition occurs, ENT is HIGH, which means that the minutes:seconds stages are at 59:59. For example, at 9:59:59, the tens of hours flip-flop holds a 0, the 74160 holds 10012 (9), and the RCO output is HIGH, putting the tens of hours flip-flop in the SET mode. The two display digits for the hours show 09. On the next rising clock edge, the BCD counter advances to its natural NEXT state of 00002, RCO goes LOW, and the tens of hours flip-flop advances to 1 so that the hours display digits now show 10 When it is 11:59:59, AND gate 1 detects that the tens of hours is 1 and the enable input is active (previous stages are at 59:59). AND gate 3 combines the conditions of AND gate 1 and the condition that the BCD counter is in the state 00012. The output of AND gate 3 will be HIGH only at 11:59:59 in the hours count sequence. On the next clock pulse, the AM/PM flip-flop toggles, indicating noon (HIGH) or midnight (LOW). At the same time, the BCD coun ter advances to 2 and the minutes:seconds stages roll over to 00:00, resulting in a BCD display of 12:00:00. At 12:59:59, AND gate 1 detects that the tens digit is 1 and it is time to advance the hours. AND gate 2 detects that the BCD counter is at 2. The output of AND gate 2 prepares to do two tasks on the next clock edge: reset the tens of hours flip-flop, and load the 74160 counter with the value 00012. After the next clock pulse, it is 01:00:00 o'clock.

Answer 1

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