Abstract
The building blocks of computing circuitry are logic circuits. Here we investigate some standard functions that will allow us to develop the architecture of a simple processing machine.
Chapter 2 is a primer of logic circuit design; examining both combinational and sequential SSI, MSI, LSI and VLSI levels of complexity. Coverage includes decoders, digital comparators, arithmetic and ALU functions, latches, flip flops, counters and registers, RAM and ROM memory, and concludes with the design of a programmable accumulator processor unit. Also described is active, open-collector/drain and 3-state output structures used to support bus architectures.
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Notes
- 1.
The LS stands for “low-power schottky transistor”. There are very many other versions, such as ALS (advanced LS), AS (advanced schottky) and HC (high-speed complementary metal-oxide transistor, CMOS). These family variants differ in speed and power consumption, but for a given number designation have the same logic function and pinout.
- 2.
For historical reasons the positive supply on logic ICs are usually designated as V CC ; the C referring to a bipolar’s transistor collector supply. Similarly field-effect circuitry sometimes use the designation V DD for drain voltage. The zero reference pin is normally designated as the ground point (GND), but sometimes the V EE (for emitter) or V SS (for source) label is employed.
- 3.
The American National Standards Institution/International Electrotechnical Commission.
- 4.
A nanosecond is 10−9 s, so 100,000,000 transitions each second are possible.
- 5.
This is called the Einstein effect. Einstein was awarded his Nobel prize for this discovery and not for his theories of relativity, as these were considered too revolutionary!
- 6.
There are around 600,000 seconds in a week and so if a cell is written into once every six seconds the entire lifetime could be used up in a week!
- 7.
If they were, then both Q and \(\overline{\mathsf{Q}}\) would go to 0. On relaxing the inputs, the latch would end up in one of its stable states, depending on the relaxation sequence. The response of a latch to a simultaneous Set and Reset input signal is not part of the latch definition, shown in Fig. 2.14(a), but depends on its implementation. For instance, trying to turn a light switch on and off together could end in splitting it in two!
- 8.
Mathematically any number can be converted to its modulo-n equivalent by dividing by n. The remainder, or modulus, will be a number from 0 to n−1.
- 9.
Strictly speaking, ROMs should also be described as random access, but custom and practice has reserved the term for read/write memories.
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Katzen, S. (2010). Logic Circuitry. In: The Essential PIC18® Microcontroller. Computer Communications and Networks. Springer, London. https://doi.org/10.1007/978-1-84996-229-2_2
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DOI: https://doi.org/10.1007/978-1-84996-229-2_2
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