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Concepts of Printed Circuit Design

Unit 3: Schematic Capture

Self Assessment Answers

Contents

 

Question 1

Question

Discuss how Grid and Snap functions in a CAD package can be used to aid schematic capture.

Answer

Grid systems are used as locating points for symbols and interconnections. To make a schematic more regular and hence, easier to follow, it is usual to place object on the sheet in a fixed manner. It is usually done with the aid of the addition of a grid on the sheet. The grid is not shown on the final diagram but is used simply as an aide to the designer to produce a design that is more pleasing and orderly in its nature. The regular grid provides visual means of assessing the clarity of the final diagram and the positioning of the symbols on the sheet. The grid dots are normally user definable.

The snap values define the resolution of the cursor movement during placement and relocation of objects. Rather than randomly placing an object on a sheet, the positions of relocated objects will be snapped to nearest coordinates, which are multiples of the snap value. The result is a much more controlled and easy to follow diagram. The snapping of nodes to a regular pattern provides a regular reference points throughout the drawing.

Question 2

Question

Logical connectivity can be useful in complex diagrams. Explain what is meant by the term and how it can be aid the designer in producing a clearer diagram.

Answer

Logical connectivity is where the connection is derived from the presence of net names or identifiers. Logical connection does not require special drawing of wires or physical contact but relies on the matching of net names that associate these objects within a single sheet or across multiple sheets of a project. The information relating to the connection is not shown on the screen or on the diagram. It is implied by the fact that the same reference name will be given to the nets that are to be connected.

A much clearer diagram results because there is not the need to show EVERY net. In a complex diagram the number of connections can result in a loss of clarity. The design can use logical connections to make the final diagram mush more readable and useable.

Question 3

Question

  1. Explain what a net is in the context of schematic capture means. How does this differ from a wire?
  2. Explain what is meant by the properties that are associated with a component in its CAD representation format.
  3. Consider why some are related to pins and others to the ‘body’ of the component.

Answer

i) A net is used to represent the electrical connection of at least two nodes. It can often be more. It is a representation that can appear on the CAD screen or may simply be stored in the design database. In some packages there is a difference between a net; the database connection and a wire; the screen representative connection.

ii) Properties are additional pieces of information connected to a primitive object such as a component, net, pin, or graphic. Properties specify information that is usually required by other design tools and is not directly relevant to the schematic capture stage of the design.

Examples include:

Different CAD packages will use properties to convey the additional information to be associated with the various schematic objects.

iii) A property is connected to the object which makes the most sense to have that attribute (such as the pin which needs a pin number attribute), but sometimes there isn't a logical place for an property to be attached (such as the device attribute or slot information attributes), so an arbitrary graphic item is used as the object to receive the attribute.

Question 4

Question

  1. Describe why buses are used in schematics rather than multiple nets/wires.
  2. The circuit shown below in diagram Q.4 illustrates a simple microprocessor system. The system has a processor, ROM memory with a 74LS373 latching chip to deal with the multiplexed Address/Data bus.
    • Sketch the circuit free hand using busses wherever appropriate.
    • Modify the circuit drawing by adding in TWO 2048 x 8 bit RAM memory chips with a normal pin configuration. Review the effectiveness of your final design and the difficulty or otherwise of the modification.

Figure Q.4: Simple Microcontroller System

Figure Q.4: Simple Microcontroller System

Answer

i) Physically a bus is a set of conductive paths that serve to interconnect the functional components of an electronic system. Electrically a bus is a collection of specified voltage levels and/or current levels and signals that allow the various devices connected to the bus to communicate and work together properly.

The problem with these large numbers of pin count is that using the traditional schematic capture techniques and working with an A4 sized piece of paper it can be difficult to fit a single component and the details of the connections onto a single sheet. The result is a diagram, which will be extremely complex and difficult to follow with ease. The use of buses in the schematic capture enables much more information to be gained in a much more simplified approach.

ii) The diagram should be fairly straightforward to draw. Even in a freehand diagram it will be much clearer and easy to modify.

Question 5

Question

Explain the key differences between the F series 74XX logic and the ALS series. Discuss why this does not affect the designer of a PCB other than at the Simulation stage of the design process.

Answer

The main difference in these types of logic (as detailed in the Course Material) can be summarised as below:

F Series Maximum clock frequency 145 MHz
  Power dissipation per gate 6mW
  Output Drive Current 20 mA
ALS Series Maximum clock frequency 45 MHz
  Power dissipation per gate 1.4 mW
  Output Drive Current 8 mA

These differences are purely operational; hence the fact that when the circuit is being simulated these variations must be taken in account. The physical packaging that the devices come in will be exactly the same. The PCB designer does not therefore have to worry as to whether the logic series if F or ALS type.

Question 6

Question

  1. Consider why certain semiconductor components can have the same reference number but a different packaging shape.
  2. Detail why some of the semiconductor packages are threaded and why the PCB designer must take this into account when placing the component on the final board.

Answer

i) Semiconductors can have the same reference numbers because they exhibit all the same electrical characteristics with the exception that the current levels are increased. The result is that for a different current carrying capacity it is necessary to adapt the packaging shape to be able to dissipate the excess energy produced. The package shapes will each have different heat dissipation characteristics.

ii) The threaded packages are usually to be attached to an extra form of heat dissipater, a heatsink. The threaded connection ensures a close and tight connection ensuring the maximum of heat transference. The final board must allow room for the additional heatsink. Components must be placed such that the heatsink will not impinge on the space required.