4. - SECOND EXPERIMENT : TEST OF FUNCTION OF THE "RESET" CIRCUIT
This circuit is very important because it is necessary before each operation on a microcomputer to reset the system to zero.
From an electrical point of view, the reset is achieved by sending a low level to the "RESET input" of the Z80.
There are two types of RESET : an automatic RESET generated by the system itself and a manual RESET obtained by pressing the RESET key. In the second case, the reset must be synchronized with the operation of the microprocessor ; the "reset circuit" allows, as we will see in this experiment, to perform this function.
4. 1. - MOUNTING OF COMPONENTS
a) Remove all the components and connections used in the previous experiment.
b) Insert on the plate the necessary components and carry out the connections represented in the Figure 6-a.
All the integrated circuits that you will use belong to the TTL family and are characterized by a higher switching speed than that of CMOS circuits. These circuits are as follows : 74LS74 (double D-type rocker), 74L121 (monostable), 74LS00 (quadruple NAND), 74LS14 (Schmitt hex-rocker).
c) Check that on the base IC3 the original integrated circuit 74C74 is in place.
The electric diagram of the circuit is given to the Figure 6-b, it is identical to that of the devices of certain computers, (more or less).
d) Position the clock generator on the frequency of 100 kHz.
4. 2. - OPERATION TEST
Let us first examine the operation of the RESET (remise à zéro) when the power is turned on. The part of the circuit which fulfills this function is composed of the resistor R3 of 10 kΩ, the capacitor C1 of 68 µF, a Schmitt trigger and the gate B. To visualize the operation of the circuit, we use in this phase L0 and L4 respectively connected to the power supply and to the output of the reset circuit.
a) By observing L0 and L4 carefully, switch on the digilab : you will notice that L4 lights up a short time after L0, that is to say that the output of gate D remains low for a short time, the time required to reset the microprocessor.
Now let's look at how the reset button circuit works. As this produces only a very brief negative pulse which cannot be observed without instruments, a circuit is used to detect the rising edges, consisting of an integrated circuit 74C74 mounted on the support IC3 of the digilab, it that is to say on the printed circuit.
b) Press the P1 button. You thus position the rising edge detector circuit ; indeed, you have just reset, thanks to this action, the toggle is at 0 ; its output Q being at 0, the LED L7 is off.
Immediately press and release the P0 button. By this action, you have just produced a negative pulse of 20 µs, pulse observed by the rising edge detector.
Look at L7 : it is on, thus indicating that the rising edge of the negative pulse caused the rocker inserted in the IC3 support of the digilab to change state.
Once the test is finished, switch off the digilab.
4. 3. - CONCLUSION
Le The examined circuit Figure 6-b) is made up of two distinct parts.
a) The first occurs when the Computer circuit is powered up. In this case, the capacitor C1 is charged through the resistor R3 for about 0.6 seconds during which the input of the Schmitt trigger is kept low. This allows the output of gate D to remain at the low level for the time necessary for resetting (RESET) of the microprocessor.
Figure 7 shows how the voltage varies at different points in the circuit.
The function of the 1N4148 diode is to quickly discharge the capacitor C1 when the power is turned off so that it is ready to be recharged as soon as power is restored.
b) The second part of the circuit, by means of which the circuit is reset to the initial conditions while the microprocessor is working, is more complex since it is necessary to synchronize the reset with the signal M1 that the microprocessor produced during its operation.
The flip-flop FF1 provides for this : on an input, is injected a square signal of frequency 100 kHz produced by the oscillator CP2 of the digilab which simulates the signal M1 normally coming from the microprocessor.
The P0 button simulates the reset key ; at rest, it is located in a position such that the input of the Schmitt TR1 trigger is at the high level. Its output is therefore at a low level and the trigger thus forces the CLEAR input of FF1 to zero, which obviously results in the RESET of this flip-flop FF1 (output Q at 0).
As soon as P0 is pressed, the output level of the Schmitt trigger goes high, releasing the flip-flop FF1. The first clock pulse which arrives on the rocker makes switch this one : its exit Q passes on the high level (since the entry D of this one was on the high level).
At exit Q, there is therefore a positive rising edge or transition which triggers the following monostable. The latter then produces a pulse of fixed duration, that is to say approximately 20 µs which, by means of NAND C and D, resets the microprocessor to zero ; in our case, this RESET pulse is observed by the pulse detector circuit and displayed by LED L7.
Figure 8 shows the variations in voltages at different points on the circuit.
Note that the start of the reset pulse is synchronized with the signal CP2 which simulates the signal M1 produced by the microprocessor.
The capacitor C2 eliminates any rebounds produced by the RESET key (in our case, the signal is already correct because the digilab button is equipped with an anti-rebound circuit).
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Immediately press and release the P0 button. By this action, you have just produced a negative pulse of 20 µs, pulse observed by the rising edge detector.
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