Synoptic Diagram of a Microcomputer and Test of the Clock Circuit of the Microprocessor :
When, in the mid-sixties, the electronics industry began to produce integrated circuits for civil use, the technique for manufacturing microcircuits was still rudimentary.
In a few years, the number of components (transistors, diodes, resistors, microcapacitors ...) included in the same box went from a few units to a few tens and then several thousand. But it was the appearance of the microprocessor in 1971 that truly revolutionized the technology.
In this practice and the following ones, we will not treat all the microprocessors one after the other, they are relatively numerous, Pentium I to Pentium IV, AMD, etc ... Consequently, we chose the microprocessor Z80 or its equivalent, the latter is very good for understanding its operation, be aware that all microprocessors operate in the same way and that the architecture is more or less complex depending on the types of microprocessors.
1. - MICROPROCESSORS AND MICROCOMPUTERS
The microprocessor is in fact no longer limited to carrying out a single task but can execute a program composed according to the needs of the user. In other words, the operations carried out by the microprocessor are executed according to a series of elementary instructions which define, instant after instant, the task of the microprocessor.
Different programs allow the same microprocessor to perform different arithmetic or logic functions, depending on the needs of each of you. For this reason, the microprocessor is used as central processing unit (in English : Central Processing Unit, CPU) in the systems managed by microcomputer.
The Figure 1 represents the block diagram of a typical system controlled by a microcomputer ; the central unit (CPU) with microprocessor constitutes the heart of the system ; a quartz oscillator allows the synchronization necessary for the successive sequence of instructions.
The programs and the data used are stored in the memory which is divided into two parts.
A part made up of a ROM memory contains the permanent data of the system which are used for the functioning of the microprocessor and which must not be lost when the power is cut, the other part is made up of a RAM memory and contains the data and programs used temporarily.
The I / O interface (Input / Output) called input / output interface in French, or I / O, allows bidirectional dialogue between the central unit and other devices called peripherals, such as keyboard or screens.
Three sets of connections, called BUS, transport the control signals, the data, the addresses of memories, thus allowing the dialogue between each subset.
Being programmable, a microcomputer system allows great flexibility of use and can therefore be connected to terminals of very diverse nature.
When the computer is used to solve management or calculation problems, certain peripherals, such as the keyboard, allow dialogue between man and machine.
The output devices most used by the microcomputer are the screen and the printer. However, it is often useful to keep the data and the programs that the memory of the microcomputer cannot keep for lack of space ; in this case, the storage is done by means of magnetic tapes or magnetic disks which are called mass memory.
When you want to send or receive information remotely, you use the telephone lines ; it is then necessary to implement special interfaces called MODEM.
The microcomputer can also be used to control industrial processes, to control robots (welding) or machine tools. In these latter cases, the terminal or peripheral members may consist of simple switches, temperature probes, pressure sensors, limit switches, etc.
The complexity of the circuits and devices connected to the central unit can vary depending on the type of application, but the structure of the system remains basically identical.
In the following practices, we have chosen the Z80 microprocessor among others and this will allow you to know all the instructions necessary for the proper functioning of the latter. Be aware that all microprocessors work in the same way, it is enough to understand binary codes as well as hexadecimal codes, the assembler ..., and the operation of the various digital circuits that surround microprocessors and its environments ...
If you have scrupulously followed our theoretical electronics lessons and the montages of digital practices at this address :Sommaire_digital.php
then this one :Sommaire_Pratique_Digit.php, then you will have no trouble understanding the process.
Once the hardware experience is acquired, we will gradually approach the development of the programs necessary for the operation of the microcomputer.
Z80 microprocessor :
It is an 8 bits microprocessor, capable of executing 158 different instructions and addressing 64 K bytes of memory.
Memories :
ROM or RAM type memories up to a maximum of 64 K bytes.
It is a circuit that can be used as a counter or as a timer to establish the sequence of instructions.
Clock generator :
It is the circuit which delivers the clock pulses necessary for the progress of a program. It is composed of a quartz oscillator which generates a square signal of 5 MHz and a divider making it possible to obtain 2.5 MHz.
WAIT circuit :
This circuit slows down the microprocessor when using memories or slow peripherals.
INTERRUPT circuit :
This circuit interrupts normal computer work to perform the work requested by a peripheral.
CPU input / output buffer assembly :
It is a set made up of three buffers used as amplifier-separators between the CPU card and the devices connected to it.
Other components :
On your keyboard for example, there are other integrated circuits which generate the control signals relating to the use of the keyboard, the PIO, the video monitor and the cassette interface, etc.
In this practice, you will examine the following CPU circuits :
the clock,
the reset circuit,
decoding of memories,
the waiting circuit (WAIT).
The experiments you will carry out will allow you to understand the principle of operation of the circuits and to test the proper functioning of the components.
To carry out the experiments planned in this practice, you will need to use the following components :
Note : You also need a PROM Programmer to program this memory. You will find the listing of this programming which will be given in hexadecimal code, that is to say the memory address entries, while the data will be in binary code in Figure 14 (output from PROM memory). For example, see the digital electronic practical lesson N° 12 at this address :Pratique/Digit_12PS1.php
1 resistance of 100 W
- 1 / 4 W at ± 5 %
1 resistance of 27 kW
- 1 / 4 W at ± 5 %
1 resistance of 220 W
- 1 / 4 W at ± 5 %
1 resistance of 22 W
- 1 / 4 W at ± 5 %
2 resistances of 10 kW
- 1
/ 4 W at ± 5 %
8 resistances of 1 kW
- 1 / 4 W at ± 5 %
1 capacitor of 33 pF
1 capacitor of 2,2 nF
1 tantalum electrolytic capacitor of 1 µF
1 tantalum electrolytic capacitor of 68 µF
1 diode 1N 4148
1 transistor BC 559 or its equivalent
1 integrated circuit 74LS00
1 integrated circuit 74L121
1 integrated circuit 74S472 (PROM) or its equivalent
1 integrated circuit 74LS14
1 integrated circuit 74LS125
1 integrated circuit 74LS393
1 integrated circuit 74LS74
5 braids of insulated rigid wire (green, red, black).
A microprocessor is a synchronous circuit, that is to say it is synchronized by means of a clock signal.
All the operations, which it performs sequentially, are punctuated by the clock signal or a signal of different frequency multiple or submultiple of the initial clock signal.
It is therefore necessary to have a stable and precise frequency clock signal, this is why quartz assemblies are used.
The oscillator generating the clock signal is part of the circuits installed on the CPU card, it comes in the form of a metal case equipped with metal pins for wiring. The frequency of the signal supplied is 5 MHz.
The oscillator circuit which supplies the clock signal is not directly connected to the microprocessor, in fact, the link is made by means of a flip-flop D and a PNP transistor as shown in the diagram in Figure 2.
The flip-flop D is mounted as a divider by 2 and makes it possible to symmetrically render the square signal produced by the oscillator. It may indeed happen that the signal produced by the oscillator is asymmetrical, that is to say that the time during which the output signal is at the high level is different from that during which it is at the low level, as shown in Figure 3.
The flip-flop "D" switches at each rising edge of the signal present at input D, that is to say at the start of each period. As the period of the input signal is very precise, the signal obtained at the output of the divider by two is of half period and also very stable. Thus, the time during which the output is high will be equal to that during which it is low, hence a duty cycle of 1.
Given the high frequency of the signal, in order to obtain correct operation of the divider circuit, a TTL technology circuit is used. But the use of such a circuit implies, given that the Z80 microprocessor is a MOS technology circuit, certain modifications will be useful in this practice.
Indeed, the clock signal necessary for the microprocessor must be characterized by a low level between 0 V and 0.8 V and a high level between 4.4 V and 5 V.
The low level of the voltage available at the output of the Flip-flop 74LS74 can be between 0 V and 0.4 V with a typical value of 0.2 V, it is therefore acceptable ; on the other hand, the high level can be between 2.4 V and 4 V with a typical value of 3.4 V, therefore not valid for controlling the Z80 microprocessor.
The solution for the signal from the Flip-flop D to be recognized by the microprocessor is to interpose a transistor adaptation circuit in order to bring the high level of the Flip-flop to the appropriate value, as the following experiment will show.
3. 1. - REALIZATION OF THE CIRCUIT
a) Make sure that the digilab commissioning switch is in the OFF position and that the power indicator is off.
b) Insert on the contact plate the integrated circuit 74LS74 (double rocker D of technology TTL Schottky), the resistances and the condenser as represented Figure 4. Do not put for now the transistor.
c) Make the connections indicated in figure 4.
3. 2. - OPERATION TEST
a) Put the digilab into service and observe the LED L0 : the lamp will be on or off depending on the position of the Flip-flop D at power up.
b)If L0 is lit, press the P0 key to turn it off.
c) Measure the voltage between earth and point A, end of resistor R2 : you must find between 0 and 0.4 Volt, that is to say a low level compatible with that requested by the microprocessor.
d) Press the P0 button so that the L0 LED is on (simulation of the CLOCK signal).
e) Measure the voltage between ground and point A ;you should find around 3.4 Volts.
You have therefore just noticed that the high level of the output voltage of the Flip-flop is unsuitable for controlling the clock input of the microprocessor.
f) Switch off the digilab and insert the transistor BC 559 in the position indicated in Figure 4.
g) Switch on the digilab and repeat the previous measurements. You notice that the low level remains the same while the high level now reaches a level between 4.5 V and 5 Volts, thus perfectly fulfilling the desired conditions.
3. 3. - CONCLUSION
In this experiment, you practically checked the functioning of the circuit allowing to adapt the signal coming from the crystal oscillator to the requirements of the microprocessor.
This circuit consists of a rocker, which divides the frequency of the square signal coming from the oscillator in two so as to make it symmetrical, and, of a transistor making it possible to adapt the levels of the signal to those required by the microprocessor.
When the Q output of the flip-flop is at the high level, between 2.4 V and 4 Volts, the output is at the low level, between 0 V and 0.4 Volt. This situation is illustrated in Figure 5.
The transistor is conductive when there is a voltage of at least 0.6 V between the base and the emitter, which is not verified in the case considered. Indeed, if Q is at a high voltage level of 4 V, the voltage between the base of the transmitter is 0 Volt.
On the other hand, in the case where the level Q corresponds only to a voltage of + 2.4 Volts, the value of the voltage present between base and emitter will be :
In the first as in the second case, the transistor is not polarized, it is therefore off ; it's as if it didn't exist. Under these conditions, one obtains at the output of the circuit for
a tension ranging between 0 and 0,4 Volt.
When on the other hand Q is at the low level (between 0 and 0.4 Volt), the voltage between base and emitter of the transistor increases up to 0.6 Volt. The transistor is therefore saturated and functions as a closed switch. In this way, the
output of the rocker is brought to + 5 Volts minus the voltage drop in the transistor (VCE of saturation). The high voltage level on
is thus around 5 Volts, a value adapted to the microprocessor.
Material Preparation
Z80 microprocessor :
output is at the low level, between 0 V and 0.4 Volt. This situation is illustrated in Figure 5.
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