逻辑器件应用要点

3.2 逻辑器件应用要点
3.2.1 逻辑器件概要
    1、TTL、CMOS、LVTTL、LVCMOS逻辑电平介绍及互连
逻辑器件的选型首先是逻辑电平的选择,在传统的单板设计中,TTL和CMOS逻辑电平被广泛应用,是数字电路设计中最常见的两种逻辑电平,LVTTL和LVCMOS是它们的低电平版本,其他的高速电频大多都衍生自TTL和CMOS。
TTL指晶体管——晶体管逻辑,由于晶体管是流控器件,且输入电阻较小,因此TTL电平的器件速度快,但功耗较大;CMOS是MOS管逻辑,由于MOS管是压控器件,且输入电阻极大,因此CMOS电平的器件速度很慢,但功耗较小,同时由于CMOS器件输入阻抗很大,外界微笑的干扰就可能引起电平的翻转,因此CMOS器件上未使用的输入引脚应做上下拉处理,不能浮空。
现实中的信号是连续的模拟信号,而数字信号只有“1”和“0”两种逻辑状态,需使用阈值来定义信号的逻辑状态。下面以图3.5以例,来说明信号的逻辑电平定义。


在图3.5中,对每种逻辑电平都定义了四个阈值,即Voh、Vol、Vih、Vil。
当逻辑器件输出高电平时,电平的幅值应高于Voh,即Voh为输出高电平的最小值;
当逻辑器件输出低电平时,电平的幅值应低于Vol,即Vol为输出低电平的最大值;
当逻辑器件输入高电平时,电平的幅值应高于Vih,即Vih为输入高电平的最小值;
当逻辑器件输入低电平时,电平的幅值应低于Vil,即Vil为输入低电平的最大值。
从图3.5可以发现,各逻辑电平的阈值并相同,因此相互间并非可以直接互连。只有同时满足一下条件时,不同逻辑电平的器件才能直接互连。
条件一、
条件二、

    2、逻辑器件的分类
电路设计中,74族逻辑器件的厂家有TI、NXP、Fairchild等。
根据工艺不同,逻辑器件可分为三类:Bipolar(双极型)、CMOS(互补金属氧化为半导体)和BiCMOS(BIPO拉人CMOS)。
Biploar是由双极型晶体管组成的集成电路,如TTL电路就属于Bipolar,特点是速度快,驱动能力强,但功耗大。
CMOS是由场效应管构成的集成电路,特点是功耗低,集成度高,但驱动能力和速度比Bipolar差。
随着高速电路的发展,单板密度越来越高,功耗越来越大,器件的工作速度也越来越快,BIPO拉人和CMOS已逐渐不能满足设计的需求,需要采用新的工艺——BiCMOS。
BiCMOS主要具有以下两种优势:
(1)BiCMOS以CMOS工艺为主,CMOS电路充当高级成都和低功耗的电路核心部分,用Bipolar电路充当输入/输入出接口部分,因此可以充分发挥Bipolar和CMOS的优势,同时具有速度高、驱动能力强和功耗低、集成度高的优点。
(2)随着工作温度和工作电压的变化,CMOS器件和BIPO拉人器件的许多特性都会发生变化,且对于许多特性参数,CMOS和Bipolar器件的变化方向是相反的。例如,传输延时参数,随着温度的降低,CMOS器件的传输延时减小,而Bipolar器件的传输延时增大;随着温度的升高,CMOS器件的擦混输延时增大,Bipolar期间的出书延时虽然也增大,但增加的幅度小于CMOS器件。BiCMOS工艺将Bipolar和CMOS这两种具有互补特性的工艺结合在一起,使得器件参数随着工作电压和工作温度的变化曲线较平缓,提高了器件的性能。
电路设计中常用的74族逻辑器件,根据制造工艺、I/O接口结构的不同,可分为多种系列。
    ABT:先进BiCMOS技术(Advanced BiCMOS Technology)
    AC/ACT:先进CMOS逻辑(Advanced CMOS Logic)
    AHC/AHCT:先进高速CMOS逻辑(Advanced High-Speed CMOS Logic)
    ALS:先进低功耗肖特基逻辑(Advanced Low-Power Schottky Logic)
    ALVC:先进低电压CMOS技术(Advanced Low-Voltage CMOS Technology)
    ALVT:先进低电压BiCMOS技术(Advanced Low-Voltage BiCMOS Technology)
    AS:先进肖特基逻辑(Advanced Schotty Logic)
    AVC:先进极低电压CMOS逻辑(Advanced Very Low-Voltage CMOS Logic)
    CBT:Crossbar技术(Crossbar Technology)
    F:快速逻辑(F Logic)
    FCT:跨苏CMOS TTL逻辑(Fast CMOS TTL Logic)
    HC/HCT:高速CMOS逻辑(High-speed CMOS Logic)
    LS:低功耗肖特基逻辑(Low-Power Schottyk L)

As the world leader in logic, Texas Instruments (TI) offers a full spectrum of logic functions and technologies ranging from the mature bipolar and BiCMOS families to the latest advanced CMOS families. TI offers process technologies with the logic performance and features needed in today’s elec- tronic markets while maintaining support for the traditional logic products. TI’s product offerings include the following process technolo- gies or device families: •AC, ACT, AHC, AHCT, ALVC, AUC, AUP, AVC, FCT, HC, HCT, LV-A, LV-AT, LVC, TVC •ABT, ABTE, ALB, ALVT, BCT, HSTL, LVT •BTA, CB3Q, CB3T, CBT, CBT-C, CBTLV, FB, FIFOs, GTL, GTLP, JTAG, I2C, VME •ALS, AS, F, LS, S, TTL Some Logic families have been in the marketplace for years, the oldest well into their fourth decade. The “Logic Migration Overview” section gives logic users a visual guide to migra- tion from the older to the newer technologies. Today’s applications are evolving with greater functionality and smaller size. TI’s goal is to help designers easily find the ideal logic technology or function they need. By offering logic families at every price/performance node along with bench- mark delivery, reliability and worldwide support, TI maintains a firm commitment to remain in the market with both leading- edge and mature logic lines. The “Product Index” section pro- vides a snapshot of TI’s extensive portfolio by function versus technology. The “Functional Cross-Reference” section shows the portfolio by device (type number) versus technology. Logic suppliers have historically focused on speed and low power as the priorities for product family improvement. As shown in Figure 1, fast performance is offered by many new TI product technologies such as AUC (1.8 V), ALVC (3.3 V) and 0 5 10 15 20 25 0 1 2 3 4 5 6 7 CMOS Voltage, VCC (V) Typical Propagation Delay, tpd (ns) HC AHC AC LVA LVC ALVC AVC AUC Figure 1. CMOS Voltage vs. Speed LV-A (5 V), depending on operating voltage requirements. Other technologies such as AUP focus on delivering “best-in- class” low-power performance. The “Packaging and Marking Information” section shows the wide variety of packaging options offered by TI. Included are advanced surface-mount packages like the fine-pitch, small- outline ball-grid-array (BGA) packages, quad flat no-lead (QFN) packages for gates and octals; and WCSP (NanoStar™/ NanoFree™) packages for single-, dual- and triple-gate functions. The new NanoStar/NanoFree WCSP packages are the world’s smallest logic packages, offering a 70% savings in space over industry-standard SC-70 packages. The “Resources” section provides additional information about TI logic families, including a list of technical literature and an overview of alternate sources for most logic families. Data sheets can be downloaded from the TI web site at www.ti.comor ordered through your local sales office or TI authorized distributor. (See back cover.)
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