DIGITAL COMPUTER ELECTRONICSThird Edition

By Malvino - Brown

PART 1: DIGITAL PRINCIPLES

CHAPTER 8: REGISTER AND COUNTERS

Registers It is the group of memory elements that work together in a unit

The simplest registers to do nothing more than to store a binary word; other modify the store word by shifting its bits left or right or by performing other operations

A Counter is a special kind of register, design to count the number of clock pulses arriving at inputs

Counters

Buffer Register It is the simplest kind of registers; all it does is store a digital word

CONTROLLED BUFFER REGISTER: The same buffer register is provided with active- high

Shift Registers Shift Register simply moves the stored bits left or right. This bit shifting is essential

for certain arithmetic and logic operations used in micro-computers

Controlled Shift Registers A controlled Shift Register has control inputs that determines what it does on the

next pulse

SHL Control

Feedback

Left Shift

Serial Loading: means storing a word in the shift register by entering 1 bit per clock pulses e.g. 4 bits need 4 clock

Parallel Loading or Broadside Loading: It takes only one clock pulse to store a digital word

Ripple Counters It is a register capable of counting the numbers of clock pulses that have arrived at

its clock input

The circuit shown is built with JK Flip flops; Since the JK inputs are returned to a high voltage, each flip flop will toggle when its clock input receive a negative edge

Ripple Counter Ripple Counter: It is known as a Ripple Counter because the carry moves through

the flip flops like a ripple in water. In other word Q0 toggle before Q1, Q1 toggle before Q2, and so on

It counts the clock pulses when commanded to do so

The problem with ripple counters is that each new stage put on the counter adds a delay. This propagation delay is seen when we look at a less idealized timing diagram

Controlled Ripple Counter

Problems with Ripple Counter

Synchronous Counters When the carry has to propagate through a chain of n flip flops, the overall

propagation delay time is ntp

For that very reason the ripple counters are too slow for some applications

To get around ripple counter we use synchronous counter

Working of Synchronous Counters However, with the Synchronous Counter, the external clock signal is connected to

the clock input of every individual flip-flop within the Counter so that all of the flip-flops are clocked together simultaneously (in parallel) at the same time giving a fixed time relationship. In other words, changes in the output occur in “synchronization” with the clock signal

The result of this synchronization is that all the individual output bits changing state at exactly the same time in response to the common clock signal with no ripple effect and therefore, no propagation delay

Controlled Synchronous Counter A low count disable all flip flops. When COUNT is high, the circuit becomes a

synchronous counter; each positive clock edge advance the count by 1

Ring CountersThe Ring Counter built with D flip flops

Application of Ring Counter Ring counters can not compete with ripple and synchronous counters when it comes

to ordinary counting, but they are invaluable when it’s necessary to control a sequence of operations. Because each ring word has only 1 high bit, you can activate one of the several devices