16.2 Registers and counters

Registers and counters are crucial components in digital systems, storing and manipulating data. They come in various types, each serving specific purposes in data storage, conversion, and counting operations.

Shift registers move data bits, while counters track and generate sequences. These versatile building blocks are essential for tasks like serial communication, temporary storage, and timing control in digital circuits.

Shift Registers

Types of Shift Registers

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• Shift register stores a sequence of bits and shifts them in a specified direction when triggered by a clock pulse
• Parallel-in serial-out (PISO) loads parallel data and shifts it out serially one bit at a time (used in serial communication)
• Serial-in parallel-out (SIPO) takes in serial data one bit at a time and outputs it in parallel once the register is full (used in serial-to-parallel data conversion)
• Universal shift register can perform both PISO and SIPO operations as well as bidirectional shifting (left or right)

Applications of Shift Registers

• Shift registers are used in various applications that require data storage, delay, or conversion between serial and parallel formats
• Commonly used in digital systems for temporary data storage, delay lines, and serial-to-parallel or parallel-to-serial data conversion
• Other applications include ring counters, pseudo-random number generators, and digital filters
• Shift registers can also be used to implement simple state machines and sequence generators

Synchronous and Asynchronous Counters

Asynchronous Counters

• Asynchronous counter, also known as a ripple counter, has each flip-flop triggered by the output of the previous flip-flop
• Propagation delay accumulates through the stages, leading to slower operation and potential glitches in the output
• Asynchronous counters are simpler to design and require fewer components compared to synchronous counters
• Ripple counter is a type of asynchronous counter where the clock signal ripples through the flip-flops, causing a delay between each stage

Synchronous Counters

• Synchronous counter has all flip-flops triggered by a common clock signal, ensuring simultaneous state transitions
• Eliminates the cumulative propagation delay issue found in asynchronous counters, allowing for faster and more reliable operation
• Synchronous counters require additional logic gates to generate the appropriate flip-flop inputs based on the desired counting sequence
• Commonly used in applications that require precise timing and high-speed counting, such as frequency dividers and event counters

Counter Types

Up and Down Counters

• Up counter increments its count value by one with each clock pulse, starting from a minimum value (usually zero) and progressing towards a maximum value
• Down counter decrements its count value by one with each clock pulse, starting from a maximum value and progressing towards a minimum value (usually zero)
• Both up and down counters can be implemented using either asynchronous or synchronous designs
• Up/down counters can count in both directions, incrementing or decrementing based on an additional control signal

Ring Counters

• Ring counter is a type of shift register where the output of the last flip-flop is fed back to the input of the first flip-flop, creating a circular shift pattern
• Only one flip-flop is in the "1" state at any given time, with the "1" state moving to the next flip-flop with each clock pulse
• Ring counters are used in applications such as time-division multiplexing, round-robin scheduling, and simple state machines
• Example: A 4-bit ring counter has four states: 1000, 0100, 0010, and 0001, which repeat in a circular sequence