Digital Registers and Logic Family Specifications

Digital Registers

As a flip-flop (FF) can store only one bit of data (a 0 or a 1), it is referred to as a single-bit register. A register is a set of FFs used to store binary data. The storage capacity of a register is the number of bits (1s and 0s) of digital data it can retain.

Loading a Register

Loading a register means setting or resetting the individual FFs—inputting data into the register so that their states correspond to the bits of data to be stored. Loading may be serial or parallel:

• Serial loading: Data is transferred into the register in serial form, i.e., one bit at a time.
• Parallel loading: Data is transferred into the register in parallel form, meaning all the FFs are triggered into their new states at the same time.

Types of Registers

1. Buffer register
2. Shift register
3. Bidirectional shift register
4. Universal shift register

Digital IC Specifications

Logic families are defined by specific parameters that determine their performance:

• Threshold Voltage: The voltage at the input of a gate which causes a change in the state of the output from one logic level to the other.
• Propagation Delay: The time interval required for a pulse to propagate from input to output.
• Power Dissipation: The power (P) required by the gate to operate with a 50% duty cycle at a specified frequency, expressed in milliwatts.
• Fan-in: The number of inputs that the gate is designed to handle.
• Fan-out: Also called the loading factor, this is the maximum number of standard loads that the output of the gate can drive without impairing its normal operation.
• Noise Margin: A quantitative measure of a logic circuit's ability to tolerate noise voltages at its inputs (noise immunity).
• Operating Temperatures: IC gates are temperature-sensitive, as they are semiconductor devices. They are designed to operate satisfactorily over a specified range of temperature.
• Speed-Power Product: A common means for comparing the overall performance of an IC family, obtained by multiplying the gate propagation delay by the gate power dissipation.

Transistor-Transistor Logic (TTL)

Transistor-Transistor Logic (TTL) depends on transistors alone to perform basic logic operations. It is the most popular and widely used bipolar digital IC family. TTL uses transistors operating in saturated mode and is the fastest of the saturated logic families.

Merits of TTL

• Good speed
• Low manufacturing cost
• Wide range of circuits
• Availability in SSI (Small Scale Integration) and MSI (Medium Scale Integration)