Electronic Troubleshooting Chapter 11 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 b 0 – b f 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 of 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 up- to 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: See Example Problem 11-1 on page 320

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 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 » 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 – 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 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 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 74LS 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 (A 15 and A 12 ) » A 12 = 0 and A 15 = 1 selects output 2Y 2 » A 12 = 0 and A 15 = 0 selects output 2Y 0

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 A 12 & A 15 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 A 12 & A 15 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