Logic adders are digital circuits that add binary numbers in either binary or binary coded decimal (BCD) format. These devices are typically used in arithmetic logic units (ALUs) and registers. There are two types of logic adders: half adders and full adders. Half adders can add two binary digits or bits. Full adders can add three binary digits or bits. The binary addition of two bits produces two numbers: the sum of the added bits and a carryover bit that is then added to the next column in the flow. Both half adders and full adders have two inputs, one for each added bit, and a single output for the sum of the added bits. Only full adders also have an additional output for handling the carryover bit.
Performance specifications for logic adders include the number of bits, density, supply voltage, operating current, and operating temperature. The number of bits is the width or size of the numbers that binary adders can process. The density is the number of independent units (flip-flops or latches) in the chip. Supply voltages range from - 5 V to 5 V and include intermediate voltages such as -4.5 V, -3.3 V, -3 V, 1.2 V, 1.5 V, 1.8 V, 2.5 V, 3 V, 3.3 V, and 3.6 V. Operating current is the minimum current needed for chip operation. Operating temperature is the full-required range of ambient operating temperatures.
Selecting logic adders 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, logic adders with emitter coupled logic (ECL) use 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 (MOSFET) to implement logic gates and other digital circuits. Logic families for binary adders include crossbar switch technology (CBT), gallium arsenide (GaAs), integrated injection logic (I2L) and silicon on sapphire (SOS). Gunning transceiver logic (GTL), a standard for electrical signals that is used to provide high data transfer speeds with small voltage swings, is also available.
Logic adders are available in a variety of IC package types and with different numbers of pins and flip-flops. Basic IC package types 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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