Transcript Troubleshooting Digital Circuit

Troubleshooting Digital Circuit
Review of logic functions
AND logic:
Review of logic functions
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):
Digital logic families
 TTL (transistor-transistor logic)
Prefix 74 on the part number
 7400 – quad 2-input NAND gate
 7404 – hexadecimal inverter
 74H – high-speed series
 74L – low-power series
 74S – Schottky transistors series, for switch on and off
faster
 74LS – low-power and schottky series, e.g 74LS00
 74AS – advanced schottky series
 74ALS – advanced low-power schottky series
 74F – fast series
Digital logic families
TTL (transistor-transistor logic)
Nominal Vcc is always 5V
TTL 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.
Digital logic families
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
Vss
Vss
Digital logic families
 CMOS (complementary metal-oxide semiconductor)
 Logic ICs with 4000 series
 4011 for quad 2-input NAND gate chip
 Motorola 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 HIGH
 If 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 cutoff
Takes 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 outputs
2. Draw the desired circuit using a CAD program
3. Define truth tables which relate inputs to
outputs
4. Describe the circuit’s operation using a
hardware description language (HDL)
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
IC packages & Identification
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).
Nature of failures
 Digital 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 HIGH
 CMOS 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 resistor
TTL shorted output: constant LOW at the output
CMOS short circuits: the result maybe a constant
HIGH or LOW, or invalid logic state.
Nature of failures
Faulty peripheral components
Numerous 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.
Potential causes of failures
 Heat-related stress
Do not stack things on top of a vented chassis
Do not block side vents
Make sure the ventilation fans are operating
Keep air filter elements clean
 High-level transient
Voltage spikes and current surges
 SCR latch-up : destroy CMOS technology
circuits
To 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 activeLOW 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 sections
Use troubleshooting flowchart (if available) or start to
troubleshoot by examining the block diagram of the
entire system
Testing methods and equipment
 Logic probes
Connected 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 pulser
Looks like a logic probe
Inject a pulse as an
input to a circuit
Used in conjunction
with a logic probe
Testing methods and equipment
Logic analyzers
Relationship between many different signals in
a complex digital circuitry
Acquire and store many channels (often 48 or
more) of logic input values simultaneously
Samples are taken at regular intervals determined by
internal or external clock source in the circuit under
test
Logic analyzer
Testing methods and equipment
Oscilloscopes
Display timing relationship between two or more
logic signals
Observe shape of two (or more) independent
signals
IC repair
 IC removal
Use 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 removal
PLCC IC chip extractor
IC repair
IC removal
Remove solder from each pin or all solder joints
must be heated simultaneously before removing
the IC chip
Never applied too much heat for too long – damage
to the circuit board
Never 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 removal
Most circuit boards cannot take a
second desoldering without being
damaged
IC repair
IC insertion
Use an IC insertion tool
Attempts 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
TOMORROW
9:00 – 11:00 : TEST
11:00 – 5:00 : Report writing
THANK YOU