Design of VLSI Systems


Posted on Feb 6, 2014

Clock signals are the heartbeats of digital systems. Hence, the stability of clock signals is highly important. Ideally, clock signals should have minimum rise and fall times, specified duty cycles, and zero skew. In reality, clock signals have nonzero skews and noticeable rise and fall times; duty cycles can also vary. In fact, as much as 10% of


Design of VLSI Systems
Click here to download the full size of the above Circuit.

a machine cycle time is expended to allow realistic clock skews in large computer systems. The problem is no less serious in VLSI chip design. A simple technique for on-chip generation of a primary clock signal would be to use a ring oscillator as shown in Fig. 5. 1. Such a clock circuit has been used in low-end microprocessor chips. However, the generated clock signal can be quite process-dependent and unstable. As a result, separate clock chips which use crystal oscillators have been used for high- performance VLSI chip families. Figure 5. 2 shows the circuit schematic of a Pierce crystal oscillator with good frequency stability. This circuit is a near series-resonant circuit in which the crystal sees a low load impedance across its terminals. Series resonance exists in the crystal but its internal series resistance largely the determines the oscillation frequency. In its equivalent circuit model, the crystal can be represented as a series RLC circuit; thus, the higher the series resistance, the lower the oscillation frequency. The external load at the terminals of the crystal also has a considerable effect on the frequency and the frequency stability. The inverter across the crystal provides the necessary voltage differential, and the external inverter provides the amplification to drive clock loads. Note that the oscillator circuit presented here is by no means a typical example of the state-of-the-art; design of...




Leave Comment

characters left:

Related Circuits

  • New Circuits

    .

     


    Popular Circuits

    Car lamp failure detector
    Low energy consumption audio signal generator
    Transistor Turns Op Amp On Or Off
    15 Watt switching power supply
    Frost Detector Temperature Sensor Circuit
    rear wheel tachometer
    Schmitt Inverter
    Secondary-Side Optocoupler Driver
    Hewlett-Packard HP-200C audio oscillator
    synthesized PLL for Low Power FM transmitter
    Dual 25W stereo audio power amplifier circuit
    Zhichuan Electronic Circuit
    Output input can be balanced equilibrium Repeater



    Top