69. Design Synchronous Rising and Falling Edge Detector Circuit

Circuit Implementation: The input I feeds directly into the first D Flip-Flop (Q0). The true output of Q0 then drives the input of the second D Flip-Flop (Q1), creating a 2-bit delay line (shift register). Combinational gates tap these outputs to perform logical operations.

Working Mechanics: 

• Positive Edge Detection: Modelled by the expression POS_EDGE = Q0.Q1'. This triggers high only when the current sample is high (1) and the previous sample was low (0).
• Negative Edge Detection: Modelled by the expression NEG_EDGE = Q1.Q0'. This triggers high only when the previous sample was high (1) and the current sample is low (0).

FPGA and CPLD Widespread Usage: Asynchronous inputs in programmable logic introduce dangerous timing violations and metastability. Passing external signals through this synchronized DFF pipeline ensures all control signals align perfectly with the global system clock.

Advantages: Extremely lightweight design footprint, eliminates asynchronous glitches, and produces a highly clean, predictable single-cycle control pulse.

Disadvantages: Introduces an inherent 1-to-2 clock cycle propagation latency delay before the output signals register the external transition.

Applications: Extensively deployed in button debounce circuits, start/stop bit identification in serial protocols (like UART/SPI), frequency measurements, and interrupt request generation modules.