Troubleshooting Digital Circuit. Review of logic functions AND logic:

Troubleshooting Digital Circuit

Review of logic functions

AND logic:

OR logic:


NOT logic:


NAND logic:


NOR logic:


Exclusive OR logic (XOR):


Digital logic families

TTL (transistor-transistor logic)Prefix 74 on the part number

7400 – quad 2-input NAND gate7404 – hexadecimal inverter74H – high-speed series74L – low-power series74S – Schottky transistors series, for switch on and off

faster74LS – low-power and schottky series, e.g 74LS0074AS – advanced schottky series74ALS – advanced low-power schottky series 74F – fast series


TTL (transistor-transistor logic)Nominal Vcc is always 5VTTL outputs are normally less than 4V, often

less than 3.5V – TTL totem pole output.Other types of output circuits used in TTL

devices are called open-collector and tristate outputs.

CMOS (complementary metal-oxide semiconductor)Complementary –

2 types of transistors in any CMOS device• n-type MOSFET: switched on (shorted from drain to

source) when a positive voltage is applied to its gate• p-type MOSFET: switched on when 0V is applied to its

gate input

Draw very little current from the power supply• Output is shorted to Vdd when HIGH• Output is shorted to Vss when LOW



Vss Vss

CMOS (complementary metal-oxide semiconductor) Logic ICs with 4000 series

4011 for quad 2-input NAND gate chipMotorola start with a 1 which is 14011

Logic HIGH input – any value greater than 2/3 of the Vdd supply voltage

Logic LOW input – any value less than 1/3 of Vdd Example: if a 5V supply is used for Vdd and Vss is ground,

Valid inputs are 0 to 1.7V for a LOW and 3.33 to 5.0V for a HIGHIf TTL outputs are used to drive CMOS, a pull-up resistor is often

added to the TTL outputs. TTL only outputs 2.4V as HIGH but CMOS requires at least 3.3V to

consider it as a HIGH input signal.


ECL (Emitter-coupled logic)Bias all transistors in such a way to keep them

between cutoff and saturation. Very fast logic device, thus used only for extremely

fast operation

TTL and CMOS use transistors either in complete saturation or complete cutoffTakes longer to switch a transistor out of hard

saturation


PLDs (programmable logic devices) To create a logic function, an engineer

specifies the input-output relationship as follows:

1. Write logic equations relating inputs to outputs2. Draw the desired circuit using a CAD program3. Define truth tables which relate inputs to

outputs4. Describe the circuit’s operation using a

hardware description language (HDL)



PLDs (programmable logic devices) A circuit board using TTL or CMOS logic chips

can often be implemented in a single PLD Unfortunately many manufacturers do not

document the relationship between the inputs and outputs of a PLD.

Undocumented PLD is impossible to troubleshoot since it is programmed by the manufacturer.


PLDs (programmable logic devices) Some common categories:

Programmable array logic (PAL) – 1 time programmable

Generic array logic (GAL) – erasable and can be reprogrammable

More sophisticated PLDs: (more logic gates and flip-flops) Field programmable gate arrays (FPGAs) Complex PLDs (CPLDs)

IC packages & Identification

Dual in-line packages (DIPs) with 14, 16, 20, 22, 24, or 28 pins

The notch and the dot to identify pin 1

General method of labeling:Manufacturer ID, part number, special

designation, package type.E.g SN74LS00N – Texas instrument (SN), TTL

(74), low-power Schottky (LS), quad 2-input NAND (00), plastic DIP(N).

IC packages & Identification

Nature of failuresDigital circuits are primarily made up of transistors.

Transistors tend to fail either in open up or short out. Open circuits – intended current flow path has been interrupted.

Too much current may have destroyed the silicon that makes up the transistor, or poor connections in IC sockets, cold solder joints, cracked PCB traces, and bent pins on the IC chip.

TTL open input: behave as logic 1 (HIGH).TTL open output: all inputs considered as as a constant HIGHCMOS open inputs: not respond to any input signal.CMOS open output: at first consider as LOW, then HIGH after some

time due to extraneous noise coupled to the input and build up a charge. Other possibility, floating input cause wild oscillation, draw more current and heat up.

Nature of failures

Digital circuits are primarily made up of transistors. Transistors tend to fail either in open up or short out.short circuits – abnormal connection of

relatively low resistance between 2 points of a circuit resulting in the flow of excess current between these points.TTL shorted input: circuit operate as a pull-up

resistorTTL shorted output: constant LOW at the outputCMOS short circuits: the result maybe a constant

HIGH or LOW, or invalid logic state.

Faulty peripheral componentsNumerous ICs require peripheral components

in order to operate.At some point, digital logic ICs are connected to

input and output circuitry such as switches, resistors, and LEDs.

Nature of failures

Potential causes of failures

Heat-related stressDo not stack things on top of a vented chassisDo not block side ventsMake sure the ventilation fans are operatingKeep air filter elements clean

High-level transientVoltage spikes and current surges

SCR latch-up : destroy CMOS technology circuitsTo prevent: CMOS inputs and outputs should never

have voltage greater than Vdd or less than Vss

Reading digital schematics

Schematics vary depending on the manufacturer

Few logic gates are physically located on a same chip but electrically distributed throughout the circuit. Each gates carry the same IC number with a suffix letter

indicating which gate it is. Numbers on the inputs and outputs identify the pin on the IC IC pins are named and numbered

A bubble is used on a logic symbol to indicate active-LOW inputs or outputs. A bar over a signal name indicates active-LOW

Troubleshooting and fault isolation

Requires a complete technical manual which includes operational procedures, block diagrams, circuit theory of operation, diagnostic troubleshooting flowcharts, and specifications.

Familiarize and understand the circuitry operation Identify and isolate faulty sectionsUse troubleshooting flowchart (if available) or start to

troubleshoot by examining the block diagram of the entire system

Testing methods and equipment

Logic probesConnected to the same

power supply as the circuit under test and has a pointed tip used to probe various points in the circuit.

To monitor the logic state of an output


Logic pulserLooks like a logic probe Inject a pulse as an

input to a circuitUsed in conjunction

with a logic probe

Logic analyzersRelationship between many different signals in

a complex digital circuitry Acquire and store many channels (often 48 or

more) of logic input values simultaneouslySamples are taken at regular intervals determined

by internal or external clock source in the circuit under test


Logic analyzer

OscilloscopesDisplay timing relationship between two or more

logic signalsObserve shape of two (or more) independent

signals


IC repair

IC removalUse an IC removal, DIP

chip extractor or a PLCC chip extractor

If using a screwdriver, pry very slightly from each end of the chip to avoid its pins from being bent.

Never use fingers and fingernails to remove a chip.

IC removalIC removal

PLCC IC chip extractorPLCC IC chip extractor

IC repair

IC removalRemove solder from each pin or all solder joints

must be heated simultaneously before removing the IC chipNever applied too much heat for too long – damage

to the circuit boardNever use a soldering iron of more than 25W. A

12W pencil iron with a fine point is best for IC work. Keep the tip clean and properly tinned

Use a desoldering tool (solder sucker). Remove the remaining solder by using desoldering braid.

Desoldering braid: braided copper or alloy ribbon

IC repair

IC removalMost circuit boards cannot take a

second desoldering without being damaged

IC insertionUse an IC insertion toolAttempts to bend the pins and insertion by hand

may cause static damage to the chip, and the pins may be curled up under the IC during insertion

Before resolder a chip, make sure any damage to the circuit board has been repaired.

Use solder sparingly and watch for bridges between pins

IC repair

TOMORROW

9:00 – 11:00 : TEST

11:00 – 5:00 : Report writing

THANK YOU

Troubleshooting Digital Circuit. Review of logic functions AND logic:

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