By carefully connecting the inputs and outputs of the D flip-flops with the clock and logic circuits, we can derive a simple stepper circuit from the figure below. The clock is fed to odd D flip-flops, and the inverted clock to even flip-flops, so this counter can be considered semi-synchronous. The explanation is based on the brilliant book But How Do It Know?, from John C. Clark, which I highly recommend.
Components:
- breadboard
- 5V power supply
- 5mm blue, yellow, and red LED
- 7404 NOT chip, 7408 AND chip, 3 x 7474 D flip-flop chip
- transistor clock circuit
(Source: referenced)
Explanation: Although D flip-flops can be in any state at startup, this state will rotate so we can freely assume that at the beginning the output of all D flip-flops, except the first one, is 0. Therefore, STEP 1 has a value of 1, and the others have a value of 0. With the arrival of the first clock, the value 1 of the first flip-flop is copied into the second flip-flop, so STEP 1 becomes 0 and STEP 2 becomes 1. When the clock is done, the inverted clock is on, so the value 1 of the second flip-flop is transferred to the third. With the arrival of the second clock, the value 1 of the third flip-flop is transferred to the fourth, so STEP 2 becomes 0, and STEP 3 becomes 1. The clock is inverted again, so the value 1 of the fourth flip-flop is transferred to the fifth. With the arrival of the third clock, the value 1 of the fifth flip-flop is transferred to the sixth, and its output leads through an inverter that resets all D flip-flops except the first one. STEP 1 has again the value of 1 and the cycle is repeated.
References:
John C. Clark, But How Do It Know? – http://www.buthowdoitknow.com/
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