Combinational Circuits : Part 2 - Static RAM, Dynamic RAM, basics of Memory Organization

Number System



  Digital Electronics : Combinational Circuits Part 2

Combinational Circuits

Here's a quick look at the concepts introduced in this tutorial

 Static RAM

SRAMs are constructed of flip-flops. The problem with the flip-flop is that it draws current all the time. Therefore, it tends to get rather warm and, on a single chip, the components cannot be packed together very tightly. The benefit is that they are very fast and are used where speed of access is important. Static RAM is often called SRAM.

Dynamic RAM

DRAMs store the information in capacitors. They are called ‘dynamic’ owing to one of its drawbacks. In use, the electricity stored in each capacitor leaks away because of the
imperfect insulation. So, after a little while the charge has to be replaced otherwise the DRAM will be empty and all the stored information will be lost. This replacing is called ‘refreshing’ and has to
be performed at intervals of about 2 ms by a DRAM control circuit. To prevent any interference with the operation of the microprocessor system, the refreshing is done in the background whenever the DRAM
is not being used. Once the static charge is stored, no further current is required (except for refreshing), therefore less heat is being generated internally and we can pack more memory into a given space. We say it
has a high packing density.

Memory organization

A memory contains a number of cells or registers that, themselves store a number of bits.
The memory organization is always quoted as ‘number of locations x bits stored in each’ so a memory can has an organization of 16x1, 16x4 or 16x8, etc.
Static RAMs usually store 8 bits in each location so a typical chip size would be 131 072x8 giving a total storage capacity of 1 048 576 bits or (1 mb). This is often referred to as 128K x 8.
Dynamic RAMs store either 1 or 4 bits in each location. One bit in each is very popular, so a typical chip organization would be 1 048 576 _ 1 which, as we can see, would actually hold the same total number of bits as the SRAM above – it’s just the organization that has been changed.

Tutorial Document ( this is a relatively short tutorial ) :

Here's a list of all the tutorials we currently have in this area - Introductory Digital Electronic Circuits and Boolean logic

 Introduction to the Number System : Part 1 
Introducing number systems. Representation of numbers in Decimal, Binary,Octal and Hexadecimal forms. Conversion from one form to the other.
 Number System : Part 2 
Binary addition, subtraction and multiplication. Booth's multiplication algorithm. Unsigned and signed numbers. 
Introduction to Boolean Algebra : Part 1
 Binary logic: True and false. Logical operators like OR, NOT, AND. Constructing truth tables. Basic postulates of Boolean Algebra. Logical addition, multiplication and complement rules. Principles of duality.  Basic theorems of boolean algebra: idempotence, involution, complementary, commutative, associative, distributive and absorption laws. 
Boolean Algebra : Part 2
De-morgan's laws. Logic gates. 2 input and 3 input gates. XOR, XNOR gates. Universality of NAND and NOR gates. Realization of Boolean expressions using NAND and NOR. Replacing gates in a boolean circuit with NAND and NOR.
  Understanding Karnaugh Maps : Part 1 Introducing Karnaugh Maps. Min-terms and Max-terms. Canonical expressions. Sum of products and product of sums forms. Shorthand notations. Expanding expressions in SOP and POS Forms ( Sum of products and Product of sums ). Minimizing boolean expressions via Algebraic methods or map based reduction techniques. Pair, quad and octet in the context of Karnaugh Maps.

Karnaugh Maps : Part 2
Map rolling. Overlapping and redundant groups. Examples of reducing expressions via K-Map techniques.
 Introduction to Combinational Circuits : Part 1
Combinational circuits: for which logic is entirely dependent of inputs and nothing else. Introduction to Multiplexers, De-multiplexers, encoders and decoders.Memories: RAM and ROM.  Different kinds of ROM - Masked ROM, programmable ROM. 
 Combinational Circuits : Part 2
 Static and Dynamic RAM, Memory organization.
Introduction to Sequential Circuits : Part 1 
Introduction to Sequential circuits. Different kinds of Flip Flops. RS, D, T, JK. Structure of flip flops. Switching example. Counters and Timers. Ripple and Synchronous Counters. 
Sequential Circuits : Part 2
ADC or DAC Converters and conversion processes. Flash Converters, ramp generators. Successive approximation and quantization errors.