In each case, choose the best option.
1 The fastest design of analogue to digital conversion is a:
(a) ramp converter.
(b) flash converter.
(c) comparator.
(d) successive approximation converter.
2 Quantization error can be reduced by:
(a) increasing the number of levels in the ADC.
(b) increasing the speed of the conversion.
(c) using vectored interrupts rather than polling.
(d) using a flash converter.
3 Using the RS232C standard, a binary 0 is most likely to be transmitted as:
(a) –4 V.
(b) +2 V.
(c) –5 V.
(d) +10 V.
4 A modem is:
(a) a type of USART.
(b) normally connected between the UART and the RS232C converter.
(c) only used in fibre optic systems.
(d) used to convert digital signals into audio tones for transmission over telephone cables.
5 This received transmission has sixteen bits of data and includes an error. It is using odd parity on the ones in a block of 25 bits:
0000111010111000100110011.
The corrected data is:
(a) 1000111010111000101110011.
(b) 0000111010111000101110010.
(c) 0000111010111000101110011.
(d) 0000111010111000100110011.
18. Test equipment and fault-finding
This chapter is intended to give some pointers towards finding faults in a microprocessor-based system. This chapter is firmly based on experience and could equally well have been entitled, ‘Mistakes I have made’.
The whole process of fault-finding should be undertaken slowly and carefully. There is a popular misconception that you have to keep busy, taking measurements, making adjustments and changing components. But, in fact, most of the time is spent just sitting and thinking (don’t forget the last two words!).
Collect the symptoms and write them down. Be wary of other people’s idea of the symptoms. If they have misunderstood what is happening you could waste hours or days going off at a tangent. If you forget to write them down, then sooner or later you will be back repeating the same checks.
In most cases, a piece of equipment or a circuit fails due to a single fault. Two simultaneous but unconnected faults are very rare. There are two popular ways of converting a small problem into a large one. These are static electricity and plugs etc.
Static electricity
When two different materials rub against each other, some negative electrons tend to migrate from one material to the other. This results in a voltage difference between the two materials. The amount of voltages can be very high – several thousand volts. If we walk across a carpet or sit on a plastic covered chair, we can become lethal to an integrated circuit designed for 5 V. Many integrated circuits have antistatic precautions built in but they have limited success. There is a trade-off here in that the better we make the antistatic precautions, the slower the integrated circuit can switch.
We can overcome the problem by reducing the build up of static by allowing it to leak away. In carpets, clothes and furniture we can do this by adding a wax or polish that absorbs and holds a small quantity of moisture. A slight dampness is a very effective way of preventing static problems. For this reason, the weather and air humidity is important. The death rate of integrated circuits tends to vary seasonally! It is not helped by air-conditioned plant where the humidity is low. The effect of static electricity on integrated circuits is difficult to predict. It generally causes small localized failures which can have very peculiar effects.
Better than spraying ourselves with water, we can take a more hightech approach but how far to go in this direction depends on what is at stake. If we are going to handle a couple of cheap AND gates once a week, then only the simplest precaution is worthwhile. However, sitting on a production line plugging in microprocessors will make any precautions economic.
The simplest method is to have a conducting band clipped around your wrist with a lead going off to a ground (earthed) point. These wristbands are made of rubber into which carbon has been amalgamated to allow it to conduct slightly. As well as the wristband we can place a sheet of this rubber on the bench top and ground the bench. Such antistatic workstations are very effective. A word of warning. Do not make your own wrist strap from a length of copper wire. This offers a very low resistance and provides no protection against electrocution in the event of accidentally touching a power line.
At home, just avoid working on a plastic table or chair or wearing clothing made from man-made fibres. Natural materials like cotton, wool and untreated wood naturally absorb some water and are fairly safe. A nice wooden bench coated with polyurethane varnish is effectively a plastic bench and should be avoided.
Problems with plugs
Many plugs used between pieces of equipment have a large number of pins. Pulling one of these out with the power connected is going to disconnect some voltages before others. This can prove fatal for integrated circuits. Either all the supplies must be on, or all should be off so never plug or unplug anything with the power on. For the same reason, never remove or replace an integrated circuit with the power on.
Are the power supplies turned on? Do you need two supplies? If you are using two supplies, are they connected together to keep their voltages in step with one another? If a ground connection is required, is it connected?
Most power supplies have floating outputs. That means that a 5 V supply, for example, will have a 5 V difference between its two terminals but neither is connected to the ground potential. This means that if we connect the negative terminal to earth, as in Figure 18.1(a), the other terminal goes to +5 V. If, on the other hand, we make the connection shown in Figure 18.1(b), the other terminal will become –5 V.
Figure 18.1 Connecting floating supplies
Have a look at the soldering if it is visible. It should be smooth and shiny. Any dull and craggy looking areas are suspect. If the integrated circuits are plugged into bases rather than being soldered, have a look to see if they have been inserted the right way round. Unfortunately, integrated circuit manufacturers take few precautions to prevent this type of error.
In most integrated circuits, the pins are numbered around the outside as shown in Figure 18.2. The position of pin 1 is always on the left-hand side of the end which has an indentation when viewed from the top as in Figure 18.2. When looking for the indentation don’t be mislead by a small circular mark where the plastic has been molded. The printed circuit board usually has either a number ‘1’ or a small square or other mark to indicate the position of the first pin.
Figure 18.2 Pin numbering of ‘dual in line’ (DIL) chips
Figure 18.3 shows the pin grid array (PGA) layout. Notice that the letters skip from H to J because of the possible confusion between I and 1. The device determines the number of pins. The one shown happens to be the elderly Intel 80386. The Pentium has 21 pins along each side.
Figure 18.3 Pin numbering of Pin Grid Array (PGA)