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Counters

 

Digital counters are integrated circuits (ICs) that count events in computers and other digital systems. Because they must remember past states, digital counters include memory. Generally, digital counters consist of bistable devices or bistable multivibrators called flip-flops. The number of flip-flops and the way in which they are connected determines the number of states and the sequence of states that digital counters complete in each full cycle. The way in which devices are clocked determines whether digital counters are categorized as synchronous or asynchronous. In synchronous devices, one clock triggers all of the flip-flops simultaneously. With asynchronous or ripple counters, an external clock pulse triggers only the first first-flop. Each successive flip-flop is then clocked by one of the outputs (Q or Q) of the previous flip-flop. Some digital counters can operate either synchronously or asynchronously. Devices can count in an increasing sequence, a decreasing sequence, or in either increasing or decreasing sequences.

Several types of digital counters are available. Binary coded decimal (BCD) counters use a 4-bit code to represent each digit of a decimal number by its four-bit binary equivalent. Decade counters are 4-bit truncated counters that produce a counting sequence from 00002 (0) to 10012 (9). Biquinary counters are decade counters in which the most significant bit (MSB) is low for five states and then high for five states. Mod-n counters use a number of different states. With non-truncated devices, the modulus (mod) number is given as 2n, where n is the number of stages (flip-flops). With truncated counters, n is the total number of different counting states. Other types of digital counters include Johnson counters and ring counters. Johnson counters create sequences that resemble shift registers and restrict the division ratio to twice the number of flip-flops. Ring counters feature a loop ring of counter stages in which only one flip-flop is set at any given time.

Selecting digital counters requires an analysis of logic families. Transistor-transistor logic (TTL) and related technologies such as Fairchild advanced Schottky TTL (FAST) use transistors as digital switches. By contrast, emitter coupled logic (ECL) uses transistors to steer current through gates that compute logical functions. Another logic family, complementary metal-oxide semiconductor (CMOS), uses a combination of p-type and n-type metal-oxide-semiconductor field effect transistors (MOSFETs) to implement logic gates and other digital circuits. Logic families for digital counters include cross-bar switch technology (CBT), Gallium arsenide (GaAs), integrated injection logic (I2L) and silicon on sapphire (SOS). Gunning with transceiver logic (GTL) and gunning with transceiver logic plus (GTLP) are also available.

Digital counters are available in a variety of IC package types and with different numbers of pins and flip-flops. Basic IC package types for digital counters include ball grid array (BGA), quad flat package (QFP), single in-line package (SIP), and dual in-line package (DIP). Many packaging variants are available. For example, BGA variants include plastic-ball grid array (PBGA) and tape-ball grid array (TBGA). QFP variants include low-profile quad flat package (LQFP) and thin quad flat package (TQFP). DIPs are available in either ceramic (CDIP) or plastic (PDIP). Other IC package types include small outline package (SOP), thin small outline package (TSOP), and shrink small outline package (SSOP).

 




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