11 Counters and Oscillators

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1 11 OUNTERS AND OSILLATORS 11 ounters and Oscillators Though specialized, the counter is one of the most likely digital circuits that you will use. We will see how typical counters work, and also how to interface data with an LED display. ounters and many other circuits often require a clock as well, and we will discuss two types of oscillator circuits that can serve this purpose. This lab will require one day. Reading: HH sections 8.03, 8.25, 5.14, 5.19 (pgs , , , ) 11.1 Binary ounter The chip is a typical 4-bit binary counter. It has several useful features: it can count up or down, it can be initialized to an arbitary value, and it has outputs to faciliate cascading multiple chips. Figure 1 shows the pin designations, with the following descriptions:, Ground: Supply voltages as usual. Q i : put bit i. Bit A is the least significant. Up: Increments counter output by one when a rising edge is received. Down: Decrements counter output by one when a rising edge is received. lear: When high, forces all output low. Load: When low, each output Q i is set to the value at input D i. arry: Gives rising edge when output wraps from 1111 to 0000 while counting up. Borrow: Gives rising edge when output wraps from 0000 to 1111 while counting down. Note that a trigger input (Up or Down) that is not being used must be held in the high state. We won t be using the D i inputs, but they should be tied to fixed values to to avoid generating errors. The Load input should be tied high. Wire up the chip with lear tied to ground and the Down trigger tied to 5 V. Drive the Up trigger with the Sync pulse from your function generator, with a frequency of 1 Hz. Monitor the outputs Q i with channels 0-3 of the digital reader, and verify that they count up at the expected rate. Drive the Down trigger instead and verify that the output counts down. B B A D A D Figure 1: Pin designations for bit binary counter. 11-1

2 11.2 LED Display 11 OUNTERS AND OSILLATORS Note that if you include the trigger signal itself as data, you really have five bits of counting capacity. If more capacity is needed, it is easy to cascade multiple chips. Put your counter back into the upward counting mode. Then get a second and wire it up like the first, but take the Up trigger from the arry output of the first chip. Monitor the four new outputs on channels 4-7 of the digital reader. Do the eight output bits correctly count from 0 to 255? 11.2 LED Display Watching the counter output on the Digital Reader is neither satisfying nor practical. Let us instead display the result on an LED numerical display. The HP display is convenient, because it has built in logic and drivers to convert a binary number to the appropriate combination of illuminated LED bars. It displays one digit in hexadecimal notation (4 bits), so you will need two displays. The pinouts for the 5082 are shown in Fig. 2. Here In1 refers to the ones-place bit, In2 refers to the twos-place bit, etc. The Latch input causes the display to hold its current value while the input is high. The Blank input causes the display to go dark while the input is high. For our purpose, they should both be tied low. Wire up both displays, using the four bits from the first counter to drive one and the four bits from the second counter to drive the other. When you run the counters, do the displays properly increment from 00 to FF? Try increasing the frequency of the clock signal. How fast can it go so that you can still tell that the numbers are counting correctly? In2 In4 In8 Blank In1 GND Latch Bottom View Top View Figure 2: Pin designations for the HP LED hexadecimal display. Note the identifier dot on the bottom of the package Binary-oded Decimal Of course, people don t usually work in hexadecimal notation. When interfacing with humans, the normal practice is to use the binary-coded decimal (BD) convention. Here counting is done in base ten, but the numbers are stored in binary representation. A 4-bit BD counter s output would thus go: 11-2

3 11.4 Timer 11 OUNTERS AND OSILLATORS 0000 (0) 0001 (1) 0010 (2) 0011 (3) 0100 (4) 0101 (5) 0110 (6) 0111 (7) 1000 (8) 1001 (9) 0000 (0) etc The four bits are not used as efficiently as in a binary counter, but in many situations bits are cheap. When you need to convert binary data to BD, you can use chips like the 74184/ (The 184 converts BD to binary, and the 185 converts binary to BD.) For small circuits, however, it is more likely to be convenient to simply work in BD throughout. For instance, the is a BD counter that is pin-compatable with the Try simply replacing both 193 chips with 192s. Does your display now count in decimal? Why you don t need to change the display chip too? If you were storing data in a RAM chip, would it matter whether it were binary or BD? One warning: encountering BD where you don t expect it can be very confusing. If you are looking at a binary signal and see the sequence , you would normally interpret it as 73 hex = 115 dec. With BD coding, however, it represents 73 dec = 49 hex. These are simply not the same, and if you don t know which coding is being used, you can t be sure what the value is. It s good to be aware of this ambiguity Timer So far we have been using the function generator signal for our clock, but if you were building a real circuit, you would not typically want to rely on an external signal. There exist a variety of ways to generate an oscillating signal of your own. One of the most convenient is the 7555 timer chip, shown in Fig. 3. The core logic is shown in part (b). Here the op amps are serving as comparators, which we will see more of in the next lab. Since there is no negative feedback, the op amp output simply rails high (5 V) or low (0 V) depending on whether the positive or negative input signal is higher. To operate as an oscillator, the 7555 is wired as in part (c). Also, the Reset signal must be tied high. The connection from the output back to the Trig and Threshold inputs causes the circuit to oscillate, at a theoretical frequency of 0.7/R. You will get to work through this calculation yourself in a homework assignment. Wire up a 7555 chip, using a 200 pf capacitor and a 100 kω resistor. Verify that the output oscillates and compare its frequency to the expected value. You should measure R and to make an accurate comparison. Replace the resistor with a 100k pot. What is the maximum frequency the circuit can produce? hoose a resistor/capacitor pair to give a signal at approximately 1 Hz, noting that the device manufacturer recommends using a larger R and a smaller to minimize the output current required. Use your timer signal to 11-3

4 11.5 Quartz rystal Oscillators 11 OUNTERS AND OSILLATORS Threshold Gnd Trig Reset Discharge Threshold ontrol R Threshold, Trig Trig (a) (b) (c) Figure 3: (a) Pin designations for 7555 timer. (b) ore logic functionality. Note that the Reset, Discharge, and ontrol signals are not shown; consult the datasheet for more information. (c) External wiring for operation as an oscillator. drive your counter, so that you have a self-contained circuit. Measure your timer frequency by counting the number of oscillations in one minute, as determined by a watch or the wall clock. How accurate is the 0.7/R formula? Two notes: First, the 7555 (and a handful of similar chips) are MOS versions of the original NE555 TTL chip, which is still available. However, the 555 chip draws a substantial current spike from the power supply when it switches, and it can be difficult to prevent that spike from affecting other parts of your circuit. The 7555 does not have this problem. Second, the 7555 and its relatives can do considerably more than just oscillate. It can be wired to act as a monostable multivibrator, the oscillation frequency can be modulated and the pulse width can be varied, among various other possibilities. If you have an electronics problem involving the generation of some kind of timed pulses, it would be worth looking through the application notes for the 555 chip for a solution. You will be using the counter and timer circuits in the next lab, so you might want to leave them set up on your breadboard Quartz rystal Oscillators It is difficult to achieve timing precision better than about 0.1% using the 7555 timer, or any other R-based circuit, due to thermal drifts in the component values. When greater precision is required, the preferred solution is the quartz oscillator. This consists of a quartz crystal that is precisely cut so that it vibrates mechanically at a particular frequency. Quartz is a piezo-electric material, so when it is subject to strain, it generates an voltage at its surface and vice versa. It is therefore possible to drive the mechanical vibration with an electronic signal. The resulting system is somewhat complicated, but it works out that the crystal acts electronically as the circuit of Fig. 4(a). Such a circuit could of course be constructed 11-4

5 11.5 Quartz rystal Oscillators 11 OUNTERS AND OSILLATORS 74H04 10M 100k 20 pf rystal 20 pf (a) (b) Figure 4: (a) Equivalent electronic circuit for a quartz crystal. (b) Oscillator circuit based on a quartz crystal. electrically, but the advantage here is that the resonant frequency is stable to typically a few parts per million. Quartz crystal oscillators are used, for instance, to generate the timing in standard wristwatches, where 1 ppm corresponds to an error of about 1 second per week. The easiest way to generate an oscillating signal using a quartz crystal is with the circuit of Fig. 4(b). It is difficult to analyze this circuit quantitatively, but the basic principle can be understood as follows: the output of the first NOT gate is fed back to its input, making an unstable circuit that inherently tends to oscillate. By passing the feedback signal through the crystal, the circuit oscillation can drive and lock to the oscillation of the crystal. The arrangement of resistors and capacitors ensures that the instability is large enough to permit oscillation but small enough for the crystal to be effective. The second NOT gate serves as a buffer, to ensure that the output signal has standard logic levels and that loads on the circuit do not affect the oscillation. Wire up this circuit using a 74H04 MOS chip and a 1-MHz crystal. Also, filter the chip s power supply pin with a 1 µf capacitor. Adjust the resistor to obtain oscillation. The signal will be clearer if you use a 10 scope probe. Measure the frequency using your oscilloscope, and compare to the expected value. (It is likely that any error you see is due to the scope, rather than the crystal.) 11-5

Due date: Sunday, November 8 (midnight) Reading: HH sections , (pgs , )

Logic Gates Due date: Sunday, November 8 (midnight) Reading: HH sections 8.0 8., 8.0 8. (pgs. 7 9, 7 ) The next few labs will deal with digital logic. In practice, you will probably find these circuits