chapter 8.pdf

Section 8.2 Latches and Flip-Flops 5738.2.5DOMultibit RegistersNOTand LatchesCOPYA collection of two or more D flip-flops with a common clock input is called aregister. Registers are often used to store a collection of related bits, such as a registerbyte of data in a computer. However, a single register can also be used to storeunrelatedDObits of data or controlNOTinformation; the only real constraintCOPYis that allof the bits are stored using the same clock signal.Figure 8-8 shows the logic diagram and logic symbol for a commonly usedMSI register, the 74x175. The 74x175 contains four edge-triggered D flip-flops 74x175with a common clock and asynchronous clear inputs. It provides both activehighDOand active-low outputs atNOTthe external pins of the device.COPYThe individual flip-flops in a ’175 are negative-edge triggered, as indicatedby the inversion bubbles on their CLK inputs. However, the circuit also containsan inverter that makes the flip-flops positive-edge triggered with respect to thedevice’s external CLK input pin. The common, active-low, clear signal (CLR_L)is connectedDOto the asynchronousNOTclear inputs of all four flip-flops.COPYBoth CLK andCLR_L are buffered before fanning out to the four flip-flops, so that a devicedriving one of these inputs sees only one unit load instead of four. This isespecially important if a common clock or clear signal must drive many suchregisters.DO NOT COPY(4)(2)1DD Q1QFigure 8-8(3)CLK Q1Q_LThe 74x175 4-bit register:DO NOT(a)COPY(a) logic diagram, includingCLRpin numbers for a standard16-pin dual in-line package;(b) traditional logic symbol.(5)(7)2DD Q2Q(6)DO NOTCLK Q2Q_LCOPYCLR74x175(b) 9CLK(12)(10)13DD Q3QCLRDO NOT COPY2(11)4 1QCLK Q3Q_L1D31QCLR75 2Q2D62Q1012 3Q3D11DO(13)NOT(15)4DCOPYD Q4Q3Q15(14)13 4Q(9)CLK Q4Q_L4D14CLK4QCLR(1)CLR_LCopyright © 1999 by John F. WakerlyCopying Prohibited