Intro to digital circuits

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Truth tables and gate symbols

Equivalent definitions of these operators can be expressed as truth tables. All possible combinations of inputs are presented, with the corresponding output. In this way, all possible states of inputs and outputs are represented. The symbol depicting the logic gate that corresponds to each operator is shown alongside the truth table. In the gate symbols, the presence of a bubble (the small circle) at an output or input means that that output or input is negated. So the symbol for a NAND gate is equivalent to the symbol for an AND gate, with the output negated, shown as a bubble at the output. Inputs conventionally appear at the left, and outputs at the right of logic symbols.
logic gates and truth tables

Clock inputs

Flip-flops and other sequential devices use a clock input. Typically, all sequential devices are tied to the same clock signal (although there certainly are exceptions to this). This ensures that the states of all sequential devices advances simultaneously. Gates are frequently mixed with sequential devices to acheive desired relationships between inputs and outputs. Sequential devices have state tables that express the next output in terms of the inputs. Transitions only occur in the presence of clock pulses. A clock input is typically shown as an input with a triangle, or the symbol CLK (or CLK’).

Connection between devices

A solid line indicates a connection from an output, or to an input. When lines intersect, a dot at the junction indicates a connection; lack of a dot indicates no connection. Power connections to logic devices are very often omitted for clarity in the diagrams. However, every gate, flip-flop, or other device will require its own connections to power and ground. Additionally, many synchronous devices have “enable” inputs (or their complements) that need to be set (or cleared) for the device to function. Other devices have reset inputs, or load inputs, or other connections which will frequently be omitted from a diagram.

Data sheets

The data sheet for a device will contain its truth table, and a pinout that shows what pins on the package correspond to the labeled inputs and outputs, as well as power and ground. VCC (or VSS for CMOS logic) indicates the supply voltage. Having access to the data sheet for a device is critical to ensure that you are using it properly. Various resources such as the GIICM, consolidate the pinouts for entire families of logic ICs into condensed reference sheets. These references can be useful, but until you have substantial familiarity, the data sheets are the best place to get your information. Data sheets for most logic devices can be downloaded as PDF documents from the manufacturer’s web sites.
Texas Instruments datasheet search
National Semiconductor datasheet search

That really wraps up the basics. If you understood this material, you are quite prepared to start building digital circuits.



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