Electronic Troubleshooting
Chapter 11
Digital Systems
Digital Systems
• Key Aspects
• Analyzing large systems based upon info in system
diagrams
• Topics covered
•
•
•
•
•
Understanding Digital Systems
Bus-oriented Systems
Problems in digital systems
Troubleshooting Digital Systems
Testing and Troubleshooting Microprocessor systems
Understanding Digital Systems
• Key Aspects
• Most digital and
computer (microprocessor
or micro-controller)
controlled systems can be
represented by a block
diagram similar to the one
on the right
Understanding Digital Systems
• Example System
•
Simple Interval Counter
• Control subsystem:
• Control Flip-Flop
• Clock
• NAND Gate –regulates
control signals
• Input Subsystem
• Reset, Start, & Stop
switches
• Arithmetic Subsystem
• Tree 7490’s arranged in a
multistage counter
Understanding Digital Systems
• Example System
•
Simple Interval Counter
• Output
• 7-Segement displays ,
drivers , and input signals
• Memory
• Lacks this subsystem
• Testing notes
• If the subsystem inputs and
outputs are known
• Subsystems operation
testable using I/O values
Bus-oriented Systems
• Characteristics
•
Bus Symbols on diagrams
• Arrow with a diagonal line and the
number of connected lines
• Replaced 7 lines between the 7447 IC and
the 7-Seg display
• Busses structures require multiple
devices to use circuits (lines) in a bus
as Inputs & Outputs
• Can cause significant problems
• Example: Gate 1 tries to pull
point X to a logic 1 and Gate 2 tries to put
it at a logic 0 – Indeterminate result
Bus-oriented Systems
• Characteristics
• Replace the gates driving
lines of a Bus with gates that
use external pull-up resistors
• Example 7401 NAND Gates
w/external pull-up resistors
• The previous using these types
of gates resolves to the gate
putting out a logic 0 controls the
line
Bus-oriented Systems
• Bus Example
• Both “X” and
“W” inputs
drive the lines
of th bus
• 4-bit data
bus
• Lines b0 – bf
• Drives
multiple
gated outputs
• “Y” & “Z”
Bus-oriented Systems
• Tristate Outputs
• Better solution
• Has the faster rise time of
the Totem Pole outputs
• Has 3 states : High, Low,
High Impedance
• Operation
• The added diode D1 will
ground out the collector or
Q2 and the base of Q3
• Q2 and Q3 are off
• No current through R3 and
Q4 is off
• Equivalent Circuit – lower
left
Bus-oriented Systems
• Tristate Outputs
• Example Circuit
Bus-oriented Systems
• Address Bus
• Used to select ICs , memory locations, multi-line
Tristate inverters or buffers (same as inverters but no
inversion of levels), etc
• Three binary address lines select one of eight outputs
• A way to select a bank of tristes to activate
Bus-oriented Systems
• Address Decoder
circuit
• Three lines on a 8-bit
address bus is used
to activate one of upto eight banks of
tristates
• Controlling the
writing data to
another Bus
• Typical circuit in Bus
based systems
• Microcomputers,
micro-controllors, etc
Problems in digital systems
• Typical Problems covered
• Ringing and Reflections
• Power Supply Glitches
• Changes in Layout, Components, and
Temperature
• Ringing and Reflections
Problems in digital systems
• Ringing and Reflections
• Caused by long interconnecting lines
• As the interconnection lines length becomes significant
to the wavelength or the signal frequency
• Load, source and transmission line impedance mismatch
can lead to signal reflections and distortions
• Reflected waves interfere with new signals on the same
line
» May be in or out
of phase with the
new signal
» End Result: the
refection combines
with the signal
forming a new third signal.
Problems in digital systems
• Ringing and Reflections
• Distortion call also occur on long interconnecting
lines due to the different impedances seen by
different components of the square wave placed
on the lines
• All interconnecting lines have distributed
capacitance and inductances
• The longer the lines the more the significant the
distributed components
• Square waves have been analyzed as consisting of
a large collection of signals with a large range of
frequencies with differing amplitudes
• They react to long transmission line according to that
analysis
Problems in digital systems
• Ringing and Reflections
• Square waves have been analyzed - continued
• Higher frequency component waveforms suffer more
attenuation than lower frequency waveforms
• Thus more distortion
• Noise pick-up and crosstalk
• Longer lines form better antennas to pick-up external
signals/noise
• Longer lines form better antennas to pick-up internal
signals from nearby lines caring other signals - aka
Crosstalk
Problems in digital systems
• Power Supply Glitches
• Sudden changes in current draw by one of several
components in a parallel connection to a
regulated power supply can cause a voltage glitch
• Caused by the voltage developed across the distributed
inductance of the line supplying the power
• Very short duration – only as long as the current draw is
changing
• Voltage spike per the following:
di
'
dt
vL  VoltageDevelopedAccrossInduc tan ce
vL  L
See Example
Problem 11-1 on
page 320
L  Induc tan ce  Henries
di
dt
 Rate  of  Change  of  Current  Amps
Second
Problems in digital systems
• Power
Supply
Glitches
• Solution
Below:
Problems in digital systems
• Ground Plane caused problems
• A large shared ground plane (as shown in the
previous examples)
• Large current draws can lead to ground level fluctuations
and related problems
• Best cure is at design time
• Provide each part of the circuit it’s own path to ground
» Thus minimizing the sharing of problems
• See Figure 11-14 on page 321
Troubleshooting Digital Systems
• Same steps as for an analog system
• Understand the circuit operation
• Apply typical inputs
• Successively split the system into smaller and smaller
sections.
• Look for circuits that have good inputs and abnormal
outputs
• Start at the approximate middle between inputs and
outputs
• Sample circuit is analyzed
• A frequency Counter
Troubleshooting Digital Systems
• Same steps as for an analog system
• A frequency Counter
Pages 324
and 325
Troubleshooting Digital Systems
• Same steps as for an analog system
• Sample circuit is analyzed
• How it works (see page 325)
• Signal to be measure is feed into the Squaring Block
» TTL compatible square wave comes out
• Before the start of a measurement – the control circuit resets
all the counters
• Then the squared input signal goes through the gate for 1
second COUNT-NOT pulses
» At the freq of the input
• COUNT-NOT pulses are feed to the counters for 1 second
» Gate is disabled to stop the counting after one second
» Counters hold the count of the number of input pulses
that occurred during the 1-second measurement period
Troubleshooting Digital Systems
• Same steps as for an analog system
• Sample circuit is analyzed
• How it works (see page 325)
• Then a store pulse is feed the Latches – enabling the storage
of the count that was on the output pins of the counters
• The latches feed the BCD to 7-Segement drivers which drive
the displays
• The clock is a 555 chip and the output from pin 3 is a
rectangular waveform that has a pulse width of 1 second
» Calibrated by adjusting pot R1
• The falling edge of the Clock (555 chip -pin 3 ) triggers the
one-shot output from pin 13 of chip 74221
» 100µsec pulse – set by R3-C2
» Enables the latches to read the output pins of the
counters
Troubleshooting Digital Systems
• Same steps as for an analog system
• Sample circuit is analyzed
• How it works (see page 325)
• The falling edge of the Latch enable one-shot pulse enables
another one-shot output from 74221 – pin 5
» Pulse resets the counters
• The count of pulses stored in the latches represents the
frequency of the input signal – since they only count during
the one second measurement period
• Timing diagram on page 327 of the textbook
• Troubleshooting the sample circuit
• Inject a testing signal of a few hundred Hz
• Watch the displays
• Normal operation is obvious
Troubleshooting Digital Systems
• Same steps as for an analog system
• Troubleshooting the sample circuit
• Watch the displays
• Indications of the source of abnormal operation can also be
discerned from the displays.
» If the least significant digit is operating correctly and the
second and third aren’t
» All the Input, timing & control circuitry is working, also IC’s 7,
10, and 13 must also be functional - check the IC supporting
the other displays
• If none of the displays are functioning normally
• Go to the middle of the circuit. Check the outputs of IC 7
• If good split the remaining part of the circuit and test again. etc
• Troubleshooting flow chart on page 328
• Large scale Integrated IC version on page 331
Testing and Troubleshooting
Microprocessor systems
• Very common to find microprocessors,
microcontrollers, Programmable Logic
devices in circuits
• Scandalized designs that can be varied to meet a
situation by changing the program in the device
• i.e., first circuit that input buttons on a WMS Bluebird Slot
Machine is a PIC microcontroller
• Programmed to de-bounce input button activations
• Some items can be checked even without a complete
understanding of such a system.
Testing and Troubleshooting
Microprocessor systems
• First Step –understand the system
• Sample system - MC6800 Microprocessor single board
system - See Figure 11-25 on page 333
Testing and Troubleshooting
Microprocessor systems
• First Step –understand the system
• Sample System - MC6800 – continued
• Has all the subsystems shown in Figure 11-1 on page 309
• MC6800 chip contains the control and arithmetic functions
• 74LS244 chip contains the Input circuits
• Tristate octal input buffer
• Connects external inputs to the data bus when enabled
» Pins 1G and 2G are active Low
• Output data leaves through the octal buffer 74LS373
• 2716 is EPROM that holds the system program(s)
• 8 data bits
Testing and Troubleshooting
Microprocessor systems
• First Step –understand the system
• Sample System - MC6800 – continued
• RAM - two 2112 ICs
• 4 data bits
• Need two for a byte of data
• Chip Select Circuit
• 74LS155 – 4 to 1 decoder
• 2- address lines are used to select one of four ICs (A15 and
A12)
» A12 = 0 and A15 = 1 selects output 2Y2
» A12 = 0 and A15 = 0 selects output 2Y0
Testing and Troubleshooting
Microprocessor systems
• Simple tests w/out maintenance
programming and special test equipment
• Usually the major components are in sockets and can
be removed
• Remove them an test surface mounted circuits that are
isolated
• On the sample circuit the MPU, RAM and EPROM can be
removed
• Obvious circuit to test is the clock
• On Sample: Pins 01 and 02 of the MC6875 and MC6800
chips
• Next check the Chip select circuits
• set MC6800 address pins A12 & A15 and check CE-NOT
pins on the other chips
Testing and Troubleshooting
Microprocessor systems
• Simple tests w/out –continued
• Major components – removed - continued
• Check the input buffers by using external data settings and
the appropriate address settings for A12 & A15
• Read the inputted data at the data lines for any of the
removed ICs
• Continue with all the supporting circuits that can be directly
tested.
• Any inverters and gates can be tested using a logic probe
and digital pulser
• Retest with some or all the major components in place