troubleshooting digital circuit. review of logic functions and logic:
TRANSCRIPT
Troubleshooting Digital Circuit
Review of logic functions
AND logic:
OR logic:
Review of logic functions
NOT logic:
Review of logic functions
NAND logic:
Review of logic functions
NOR logic:
Review of logic functions
Exclusive OR logic (XOR):
Review of logic functions
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
Digital logic families
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
Digital logic families
Digital logic families
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.
Digital logic families
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
Digital logic families
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)
Digital logic families
Digital logic families
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.
Digital logic families
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
Testing methods and equipment
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
Testing methods and equipment
Logic analyzer
OscilloscopesDisplay timing relationship between two or more
logic signalsObserve shape of two (or more) independent
signals
Testing methods and equipment
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