東京大学 情報理工学系研究科 創造情報学専攻 2012年8月実施 筆記試験 第2問

Author

itsuitsuki

Description (English)

Design a logic circuit to light an LED satisfying the following conditions. Figure 1 shows the connection diagram of the circuit, input signals and an output signal to the LED.

Condition 1: Inputs of the circuit to design are LEDSTR and CLOCK INPUT ( clock input signal). The output of the circuit to design is LEDOUT. LEDOUT is directly connected to the LED driver and lights the LED. When LEDOUT is H, LED is on, and when LEDOUT is L, LED is off.

Condition 2: LEDSTR (INPUT 0, INPUT1, INPUT2) specifies the strength of the LED lighting. When LEDSTR is 0, the LED is completely turned off. When LEDSTR is 5, the LED is continuously turned on. When 1 <= LEDSTR <= 4, the strength of the light from the LED is proportional to the value of LEDSTR. When LEDSTR is more than or equal to 6, any behavior of the circuit is allowed.

Condition 3: When the LED turns on and off repeatedly at more than , the strength of the LED light is seen as an average value of the time periods when the LED is turned on.

Condition 4: The logic circuit is designed using AND, OR, XOR, NOT and D-type Flip-Flops.

Design the circuit following the questions below.

1. Design a 3-bit counter that count from 0 to 4 repeatedly such as

2. Design a circuit that compares two 3-bit numbers.

3. Design a circuit to turn on the LED for 1 clock period when LEDSTR is 1, 2 clock periods when LEDSTR is 2, ......., 5 clock periods (i.e. always on) when LEDSTR is 5. Figure 2 is an example of the output waveform.

4. Using the logic circuit designed in (3), design a logic circuit that turns on and turns off the LED once for more than a second with the strength of the light specified by LEDSTR.