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CLOCKS AND TIMING CIRCUITS

7. CLOCKS AND TIMING CIRCUITS. LEACH, MALVINO & SAHA. Clock Waveforms. Ideal clock waveforms. Synchronous Operation The low to high transition is frequently called the positive transition(PT). A circuit that changes state at this time is said to be positive-edge-triggered

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CLOCKS AND TIMING CIRCUITS

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  1. 7 CLOCKS AND TIMING CIRCUITS LEACH, MALVINO & SAHA

  2. Clock Waveforms • Ideal clock waveforms. • Synchronous Operation • The low to high transition is frequently called the positive transition(PT). A circuit that changes state at this time is said to be positive-edge-triggered • The high to low is called the negative transition (NT). A circuit that changes state at this time is said to be negative-edge-triggered. • Virtually all circuits in a digital system are either positive-edge-triggered or negative-edge-triggered, and thus are synchronized with the system clock.

  3. Clock Waveforms • Characteristics • Propagation Delay Time • Propagation delay tp is the time between a PT (or a NT) at the input of a digital circuit and the resulting change at the output. • Propagation delay = tp ≈ tpLH ≈ tpHL

  4. Clock Waveforms • Pulse-Forming Circuits

  5. TTL Clock • A 7404 hexadecimal inverter can be used to construct an excellent TTL-compatible clock.

  6. Schmitt Trigger • A Schmitt trigger is an electronic circuit that is used to detect whether a voltage has crossed over a given reference level. It has two stable states and is very useful as a signal-conditioning device. • The value of Vi that causes the output to jump from low to high is called a positive-going threshold voltage VT+ • The value of Vi causing the output to switch from high to low is called the negative-going threshold voltage VT- • The difference between the two threshold voltages is known as hysteresis Schmitt-trigger transfer characteristic. Schmitt trigger inverter.

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