Today, almost every electronic device we encounter operates based on a sequence of operations controlled by a microprocessor. Such microprocessor-based devices can be found in almost all fields: automotive, health, manufacturing machines, household appliances, children's toys, etc.
In the manufacturing industry, many manufacturing processes (or parts of processes) are carried out by robots, which are microprocessor-based systems. This type of robot is usually called an Industrial Robot.
The problem of maintaining microprocessor-based systems can basically be divided into 2, namely embedded microprocessor systems and those that are not attached to the system (eg personal computers). Some damage tracking techniques for both types of microprocessor systems are the same, but there are unique tracking techniques for Personal Computer (PC)-based systems.
1. Basic Concepts & Structure of Microprocessors
A microprocessor is a device in the form of an IC, can be programmed and functions as a digital data processing center, or a logical decision maker and can affect the operation or performance of the system. Therefore, this IC is often called a central processing unit or Central Processing Unit (CPU).
Figure 9.1: CPU in a Microcomputer
2. Basic Principles of a System in Engineering
In the system, the input quantities that enter the system will be processed into outputs that are issued through output terminals, as shown in Figure 9.2. Larger and more complex systems are usually depicted & simplified in the form of block diagrams.
Figure 9.2: Understanding Engineering Systems
3. Basics of Microprocessor-Based Systems
Control of a system that works steadily and continuously can be done by a microprocessor. If the control is desired to work automatically, then it is necessary to use feedback that will provide information to the microprocessor about what it controls.
Figure 9.3: Basic Microprocessor-Based System
Microprocessor input can come from input devices (switches, sensors, etc.), while microprocessor output is in the form of instructions to activate actuators or move control circuits. Some products store these programs in ROM permanently, for example programs for children's toys, car starters, robots in the manufacturing industry, etc. Figure 9.3 shows the basis of a microprocessor-based system.
Microcomputer Based System Maintenance
The classification of a computer, mini or micro is not determined by physical size, but rather emphasized on the number of functions that can be performed and the speed of processing data and the memory capacity it has. A microcomputer generally consists of a microcomputer IC on a PCB (printed circuit board), a ROM containing a program (usually an operating program) that is only a few bytes in size (256 bytes), and a RAM containing data. Compared to a PC computer, the size of the RAM and ROM of a microcomputer is smaller, so the programs that can be stored are limited. A microcomputer also has a master clock from a crystal and several other ICs to form special functions and handle operations on all I/O ports (Input output ports). The I/O port of a microcomputer is also equipped with a UART (Universal Asynchronous receiver/transmitter) which produces a standard interface to the printer.
Figure 10.1. Microcomputer Block Diagram and Output Devices
1. CPU
It is the central IC for data processing. It is the heart of a computer. The CPU also contains a clock to drive the logic inside the computer.
Figure 10.2: Example of a PCB from a computer
2. Memory
It is a component that can store information or programs. Programs to run a computer (usually called an operating system) are stored in ROM (Read Only Memory). Programs stored in ROM are permanent (made by the microcomputer manufacturer), they are not lost even if the computer is turned off. While programs created by the user will be stored in RAM (Random Access Memory). If the computer's power supply is turned off, the program or information in RAM will be lost.
3. I/O Ports
It is a chip designed for communication between devices inside the microcomputer (in the striped box) and external devices (printers, monitors, interfaces, etc.).
4. Program
It is a series of binary numbers that will run the machine. This type of program is called machine code or object code. A program written by a programmer is called a source program or source code.
Source programs written in mnemonics are called assembly programs. Mnemonics are alphanumeric symbols for binary instructions that are understood by machines (CPU). Programs written in the form of language instructions that are understood by humans are called high language programs. In order for this program to be understood by the machine, it must be translated. Translators of high language programs to machine language are called translators or compilers. High language programs can also be processed directly by the interpreter so that the instructions can be directly understood by the machine. Interpreters work faster than compilers or translators.
Application programs are programs that are used to perform certain tasks, for example processing words (e.g. Word processor), or processing numbers (e.g. Excel), and so on.
The Control Program is a program used to load application programs or data on the computer.
A machine cycle is the time required to execute an instruction. Execution of an instruction may sometimes require several machine cycles.
Principle of Microcomputer Operation
All microprocessors (the heart of the computer) have at least two types of machine cycles:
- Reading cycle
- Writing cycle
1. Reading cycle
During a read cycle, the CPU reads a memory location (RAM), then places the address of the memory location on the address bus. After the address is received, the memory places the data stored in it onto the data bus. The CPU then latches the information at the end of the cycle.
2. Writing cycle
During a write cycle, the CPU writes data to a memory location. The address is placed on the address bus lines, and at about the same time, the data is placed on the data bus lines, and the RAM latch-ons the information at the end of the cycle.
3. Control path
One or more control lines will notify the memory, I/O chips and peripherals when the read and write cycle is complete and the data has been latched.
Currently, many devices use microcomputers, for example TV tuners, automatic washing machines, robots, children's toys, machine systems in cars, process control devices in industry (food industry, oil industry, textile industry, and so on).
Example of Digital Circuit Damage Case
To further clarify what has been explained above, an example of a digital circuit is provided below.
1. First series
It is a series of flashing lights with memory as in Figure 5.35.
Figure 5.37: Blinking Light Circuit with Memory
How the circuit works is: This circuit uses a CMOS IC so that the current taken is very small (efficient). There are two important parts in this circuit, namely for gates C and D to work as the simplest one-bit memory circuit (RS FF). While gates A and B work as low-frequency oscillator circuits. If the trip switch is pressed, pin 8 gets low (logic 0) for a moment so that pin 10 will be high (logic 1) continuously (memory) until the clear switch is pressed, then pin 10 will be low. When pin 10 is high, the oscillator circuit works so that the output from gate D will change in the form of pulses (alternating logic 0 and 1) and this is used to turn on/off the transistor alternately, so that the LED also flashes on and off. The frequency of this circuit is determined by the size of C1 and R3, the smaller the value of C1 and R3, the higher the frequency. If this circuit is to be modified into an alarm circuit, the value of C1 or R3 is changed to a smaller value {can be tried or use the formula to find the frequency f? 0.7 / (R3.C1) Hz} and the LED is replaced with a speaker.
Before studying the damage to this circuit, we must first know what logic is present at the output of each gate when working normally, namely:
Logic condition A is the logic condition after the trip switch is pressed momentarily. Logic condition B is the logic condition after the clear switch is pressed momentarily. 1/0 or 0/1 is the pulse condition seen with the logic probe.
For some of the damages below, we will study them through the existing data.
a. Damage 1: measured with the logic probe on the IC legs after the trip switch is pressed momentarily, as follows:
From the data above, it is clear that the RS FF circuit has no problem, so the problem is that the oscillator circuit is not working, it only functions as gates. So the component that makes the oscillation is damaged, namely R3 is open or C1 is short-circuited.
b. Damage 2: measured with the logic probe on the IC legs after the trip switch is pressed momentarily is as follows:
[
From the data above, it is clear that the FF and oscillator circuits are working well. So the only remaining final circuit is a switch circuit with a transistor that is likely damaged because the base leg should be the same as the 4th leg of the IC. For that, of course, the most suspected damage is R4 open or the transistor is damaged base and the emitter is short circuited.
c. Damage 3: The LED will stay on without blinking after the trip switch is pressed for a moment, but if the clear switch is pressed for a moment, the LED will go off again. From the data above, it is clear that the FF circuit works well, but the oscillator circuit does not work only as a passer of ordinary gates. So the damaged component is C1 open or R6 open.
So just by using a logic probe tool we can analyze a simple digital circuit from its operation until there is damage to the circuit.
2. Second series
The generator ramp circuit is as shown in Figure 5.38 below:
Figure 5.38: Ramp Generator Circuit
The way this circuit works is: a digital ramp generator, built from IC 7493 (4-bit counter) with the addition of an R-2R ladder network. This network is commonly used in DAC circuits. This circuit uses TTL which produces a 16-step ramp output. The schmitt trigger-based oscillator produces pulses to increase the 4-bit binary counter (7493). This counter divides the input frequency by 2, 4, 8 and 16 so that a 16-step waveform will appear at the output of the R-2R ladder network. The oscillator produces about 1KHz so that the ladder waveform can be easily observed.
This ramp waveform is widely used in many instruments and measurements that usually require good linearity. So its normal condition can be seen with an oscilloscope on each Tp. Where TP1 is in the form of a square wave pulse as the sender of the ramp circuit pulse, so that the output is a 16-step ladder shape (see Figure 5.38).
Some of the damages we will review below:
a. Damage 1: the output frequency is obtained twice but the ladder shape is only 8 steps as in Figure 5.39.
Figure 5.39: 8 Step Stairs
Here it can be seen that one step is missing so that the output changes to only 8 steps with a frequency twice the normal, namely the last step (leg 11 7493) is not connected, so the damage is definitely R8 is open.
b. Damage 2: a square wave appears at the output with the same frequency as the ramp frequency. Obviously as long as the output is still there even though it is wrong then the IC 74123 or 7493 still works, so only in the circuit outside the IC. Because it only becomes one pulse in the same time as the ramp, the R7 section is open because the ladder function becomes non-existent (legs 8, 9, 12 do not appear at the output).
From the damage above, it can be concluded that when R7 is damaged, the number of steps at the output will change but the frequency remains normal, while for R8 damage, both the number of steps and the frequency will change.
Summary
- There are various types of digital ICs, namely: RTL, DCTL, DTL, TTL, ECL, CMOS, LOCMOS, PMOS, NMOS, IIL, SSI, MSI and LSI, each of which has different characteristics.
- The digital ICs that are widely used in general circuits today are TTL and CMOS ICs.
- The memory circuit on a digital IC (Flip-Flop) can be used to create counter and register circuits.
- Auxiliary equipment for finding damage in digital circuits, besides multimeters and oscilloscopes, is usually quite specialized, such as: logic clips, logic probes, logic pulsers and digital IC testers.
- Techniques for tracking digital circuit failures are: look and touch, heat and cool, IC stacking, approaching similar ICs and very careful measurements.
Chapter 5 practice questions
- Mention the various types of digital ICs that exist!
- What are the advantages and disadvantages of TTL IC compared to CMOS?
- Explain the working of a counter circuit and create a circuit to be able to count up to 16 decimals. How many ICs are needed?
- What is the use of the logic probe? Explain how to use the measuring instrument correctly.
When to use the technique of tracking damage to digital circuits by:
- a. heat and cool
- b. IC accumulation
Group task
By forming groups of 3 people each, work on the following tasks by discussing them:
By looking at the circuit diagram 5.37 on page 5-24, try to analyze the problem that occurs and determine which component is damaged, the type of damage and the reason, if: the output becomes only 4 steps but the frequency does not change.
Damage Tracking Tool
Before tracking damage to a digital circuit, you need to know first the tools that are often used to make it easier to find damage. Some tools are very rarely used in tracking analog circuits, except for multimeters and oscilloscopes so you must first understand the function and how to use the tool. The tools are:
1. Multimeter
There are two types of multimeters commonly used, namely analog multimeters and digital multimeters (figure 5.21). All can be used for measurements on digital circuits, but since the release of DMM (Digital Multi-meter) technicians prefer it because of its better capabilities, suitable for testing electronic circuits and more accurate. This digital meter has characteristics: high input impedance, so it does not damage the digital circuit, with voltage and current much different compared to analog circuits. So that testing digital circuits without fear of inaccurate readings caused by overloading the circuit, or circuit fussiness caused by oversized test equipment.
Figure 5.21: Analog Multimeter and Digital Multimeter
2. Logic Clip
Logic clip. A digital circuit test tool, shown in Figure 5.22. This easy-to-use tool exposes the pins on the top. A measuring or monitoring device or small clip can be connected to the pins to determine the logic levels at the pins of the device under test.
Another type of logic clip has built-in monitoring capabilities (Figure 5.23). In addition to the pins shown, the top of the clip contains two light-emitting diodes (LEDs), which continuously display the logic state of each pin on the chip. If the LEDs are lit (indicating a logic 1) by power from the circuit under test. All pins are electrically supported so that the clip does not interfere with the circuit under test.
Caution: When using a logic clip, turn off the circuit power, connect the clip and then turn on the power. (This helps prevent short circuiting the chip.)
Figure 5.23: Logic Clips Provide Visual Indication of Pin Logic Conditions
3. Logic Probe
If you want to really get into the circuit, you can use a logic probe. A burned chip can't be repaired, but a logic probe can tell you which chip is fussy so you can replace it.
The logic probe shown in Figure 5.24 is a widely used tool for this type of analysis. Logic probes cannot perform some of the complex test equipment tasks that logic analyzers can. However, the frequency of chip failures in electrical circuits is high. The simplicity of the probe and its ability to quickly track down faults in live circuits make it an ideal tool for 90% of fault isolation needs.
When the pointed tip of the probe is placed on a pin of a suspected faulty chip, a test point or trace on a circuit board with an indicator light near the tip of the probe will indicate the logic level of that point. The metal tips of most logic probes sold today are protected against damage from high voltage (up to 120 Volts AC for 30 seconds) from logic gates (+5 volts).
Some probes have two LEDs mounted near the tip, one for logic HIGH and one for logic LOW. Better probes can also tell if the test point has a pulse. They can also store short pulses that occur to tell if there is a glitch or spike at that point.
Figure 5.24: Types of Logic Probes and How to Measure Them
If you are going to buy a logic probe, make sure that it works with the logic chip family you are analyzing. The ability to touch a point with the probe tip and determine the state of that point directly for diagnostic analysis and its ability to store pulses make this tool easy to use and widely accepted as a suitable diagnostic tool for all but the most complex digital fault detection.
Another advantage is that Logic probes can display the logic state near the probe tip itself, whereas other devices force you to pull the probe measurement and then turn to some display to see the state.
The logic probe in Figure 5.24 gives four indications:
- The first red LED is for logic LOW (logic 0).
- Green LED for logic HIGH (logic 1).
- The second red LED is for floating or tri-state.
- The third red LED (yellow LED) is for the pulse signal.
Power for the probe comes from a clip connected to a voltage on the circuit under test. The other clip is connected to ground providing improved sensitivity and noise immunity. The probe is ideal for finding short-duration, low-frequency pulses that are difficult to see with an oscilloscope but is more often used to quickly locate a gate whose output is hung or locked, in a HIGH or LOW state.
A useful method for circuit analysis is to start the probe at the center of the suspected circuit and check for a signal. (This assumes, of course, that you have and can use a schematic.) Move back or forward toward the faulty output as shown in Figure 5.25. It won't take long to find the faulty chip whose output remains unchanged. A limitation of the logic probe is the inability to monitor more than one line.
Figure 5.25: Circuit Analysis Starting at the Center of the Circuit
4. Logic Pulse
If the circuit under test does not have pulses or changing signals, controlled pulses can be introduced into the circuit by using a logic pulser (Figure 5.26). This easy-to-use tool is a portable logic generator.
Activated by a button or slide switch, the pulverizer senses the logic level at the point touched by the tip and automatically generates a pulse or series of pulses of the opposite logic level. The pulses are visible on an LED light mounted on the pulverizer handle.
Its ability to introduce a signal change into a circuit without unsoldering or cutting wires makes the logic pulser an ideal combination with the logic probe. The two devices used together allow step-by-step evaluation of the response of a circuit section.
Figure 5.27 shows several ways to test logic gates using probes and pulses. Assume the output of the NAND gate remains HIGH. Testing inputs 1, 2, and 3 shows that all are HIGH. This causes the AND gate output to go HIGH, resulting in the NAND gate output being LOW. Something is wrong. Placing a probe at the AND gate output produces a LOW output. It should be HIGH. Now which gate is bad?
To find it, place the probe on the NAND output (gate B) and the pulse on the AND output (gate A NAND input gate) as shown in figure 5.28.
Figure 5.26: Logic Pulser That Can Provide Signals to a Circuit
Figure 5.27: Several Ways to Test Logic Gates
Figure 5.28: Place the probe at the output of the NAND gate and the pulser at the output of the AND gate
Figure 5.29: Place the Probe and Pulser at the Output of the AND Gate
Give this line a pulse, the probe should blink. Indicating a change in the input to the NAND. If there is no change, the AND may be faulty. But is the LOW caused by the short to ground at the AND output or the AND input? Place both probe and pulser at the AND output, trace as shown in Figure 5.29 and give this line a pulse. If the probe blinks, the NAND is faulty, the input is changed so the output state can change as well.
If the probe does not blink, you know that the line is shorted to ground. One way to determine which chip is shorted is to touch the chip case. A shorted chip will feel warm, while a hung chip will appear to be normal but will not change its state.
5. IC Tester
Advanced fault detection equipment has become very sophisticated (and expensive). You can now buy equipment that can test almost every chip in the system.
Micro Sciences, Inc. in Dallas Texas, created an IC tester that could test over 1007400 TTL and 4000 CMOS electronic circuits. This testing capability included RAM and ROM chips.
Microtek Lab in Gardena, California, built a tester that works perfectly as a functional pin tester for 900 devices on the TTL 54/74 series of chips. This tester displays the state of the chip under test on a liquid crystal display (LCD) as shown in Figure 5.30. The device uses LEDs to signal GO/NO GO, as the test results.
Figure 5.30: IC Tester
6. Oscilloscope
Oscilloscopes have been around for many years, although in recent years they have evolved, adding a number of capabilities. An oscilloscope is an electrical display that can plot a graph of a voltage signal's amplitude versus time or frequency on a CRT screen (Figure 5.31). A scope (short for oscilloscope) is used to analyze the quality and characteristics of the electrical signal sensed by a probe touching a test point in a circuit. The scope is also used as a measuring instrument to determine the voltage level of a particular signal.
Figure 5.31: Types of Oscilloscopes
There are many types of oscilloscopes available today, from single trace to seven digital channels with various colors. There are also digital oscilloscopes with memory that can store and even print out the results. In addition to sensitivity and trace/channel display, one major difference in the characteristics of oscilloscopes is in terms of the ability of the receiver frequency width (bandwidth). This varies between 10 MHz to 300 MHz and the price is according to the frequency width.
An oscilloscope is a useful tool for monitoring analog signals or signal variations and displaying static waveforms on a CRT screen that is limited by a measurement grid. Oscilloscopes are very useful in analysis, you can not only measure the voltage, amplitude, and frequency of the signal being tested, but can also measure delay time, signal rise and decay time and even localize occasional glitches.
The cool thing about dual-trace, quad trace, and even lighttrace is the ability to view different signals simultaneously. For example, you can look at the input and output of a gate and measure the time delay between the input and output signals. Another useful technique is to simultaneously view all or part of the data bus / address bus to see the logic levels (HIGH = +5 V, LOW = 0 V) and what binary numbers they represent.