ECEN 1400, Introduction to Analog and Digital Electronics Lab 7: Seven segment display 1 INTRODUCTION In this lab, you will wire your two counters, each with custom roll over levels, to two seven-segment decoders which in turn will drive two seven segment displays. This circuit has many of the features of a (primitive) clock. For your convenience, questions and lab procedures are introduced in a unique color. 2 COMPONENTS AND TOOLS REQUIRED From your kit: o Breadboard o Wires o Wire-cutter and pliers o A pre-assembled 4-bit 74161 counter circuit driven either by the function generator or a 555 timer. From your TA, two of each of the following chips. The datasheets are in the reference section of the website. o (2 of) 7447 seven-segment display decoder chip o (2 of) seven segment display chip On the lab bench: o Variable DC power supply o Function-generator if you don t have a 555 oscillator feeding your counter. o Oscilloscope 3 CONFIRM YOUR BCD COUNTER In this section, you will drive four of the seven segments directly from your counter as an intermediate step to the full decoder/display circuit. This will confirm your counter is working as designed and that the display chip will work. Get the four chips from the TAs and place all four of them on the breadboard. You will only use the seven-segment LED chips in this section the decoders will be used in the following step. Include room for an AND chip and an inverter (from the previous lab) after the counters. Examine your circuit layout and consider how to arrange all of the new chips so the physical circuit is easy to understand and debug. Use the color of your jumper wires to mean something red is Vcc, black is ground etc. Keep the wires tight to the breadboard with the minimum of overlap. Consider arranging the breadboard in similar to your circuit design, just to make it easy to understand. Version 1.1, 8/25/13 R. McLeod from earlier versions 1
The seven segment display chip contains 7 LEDs, similar to the discrete devices you have been using. The difference is that they have been packaged into a single chip, and they have a different shape. Notice that the bottom of the chip has only 10 pins. We know that each LED has an anode (where the current goes in) and a cathode (where the current comes out), which would imply that there should be at least 14 pins. Since the individual LEDs are supposed to be used together, they are wired so their anodes (the + ends) are all tied together internally, as shown in Figure 1. a f Top view Vcc (5V) b e g c decimal d Figure 1. Pinout of a common-anode seven segment display. This wiring arrangement is known as the common anode configuration. These ends of the LEDs are connected to the upper-right pin of the display chip (when looking at it from the top). This is shown schematically in Figure 2. The upper-right pin of the chip should be connected to 5 V. The remaining pins are connected to the cathode ends of the LEDs. Note that these LEDs are wired in parallel, which we discussed previously was a bad idea. However, these diodes are all fabricated at the same time on the same substrate and are thus very close to identical, which avoids the problem of wiring discrete, variable LEDs in parallel. Figure 2. Internal wiring diagram of a common-anode seven segment display. Version 1.1, 8/25/13 R. McLeod from earlier versions 2
Figure 3. Step 1 Use your seven segment display as a bank of LEDs. Note that ABCD are tied to ground. Connect the second counter s clock to the QD line of the first counter to make, effectively, a single 8-bit counter. Connect each of the four binary outputs of both of the 161 counters (QA,QB,QC,QD) to current limiting resistors and then to the cathode pins of the seven segment display. Set the function generator to a 0 to 5 V square wave with about 1 Hz frequency. Verify you have two, 4-bit counters. You now have 8 LEDs, albeit in a strange package, counting from 00000000 2 to 11111111 2 = 255 10. Note specifically how a particular binary clock output appears on the display (hint: this is a common anode chip, which matters). Every 16 seconds (if you have a 1 Hz input) the first counter should roll over and the second counter should increment. Watch long enough to confirm that you have the circuit operating correctly. You do not have to take any particular data, but do note your observations. 4 INSERT THE DECODERS Now you will change from binary output (eight LEDs blinking on and off) to decimal output (two digits formed from seven segments each). The logic circuits required to Version 1.1, 8/25/13 R. McLeod from earlier versions 3
generate the seven signals that represent the decimal equivalent of the four bit input have been gathered into a single chip, the 7447 decoder. This device is used to translate, or decode, a binary number from the 4-bit binary representation to a 7-bit code. The 4-bits of the binary number are inputs to the decoder while the 7 bits that control individual LED segments are the outputs of the decoder. Connect the four outputs of each counter to the four inputs of each decoder as shown in Figure 4. Add three more current limiting resistors to each bank so that you can connect the seven outputs of the decoder to the seven inputs of the display chip. Figure 4. Step 2 Include the decoders. Observe the operation of 4-bit counters driving seven-segment displays. You should now see the decimal digits counting quickly on the first counter and slowly on the second. However, the digits will occasionally look strange. Figure out why and confirm that you are seeing the right pattern by consulting the datasheet on the references page of the website. 5 ENFORCE A CUSTOM ROLL OVER CONDITION ON BOTH COUNTERS Version 1.1, 8/25/13 R. McLeod from earlier versions 4
Note a portion of this was an extra credit problem in Lab 5. If you already did it, you re ahead here skip directly to connecting the display. The strange symbols in the last section were due to the binary input not being limited to the set of values that represent a binary-coded decimal (BCD) number. Now consider the two 74161, four-bit counters on your breadboard as representing two BCD digits of a two-digit number such as the number of minutes on a clock. The first counter should have its clock driven by the oscillator and should roll over when it reaches a value of 9 10. That is, the counter should start at 0000 2 and count to 1001 2, then begin again at 0000 2. For your convenience, the complete circuit is shown below. In your lab report, show why the NANDs (which you can implement with ANDs and a NOT) are the right circuit to implement the desired functions. Implement a 0-9 counter. Design a single-gate logic function to detect the roll over condition when DCBA reaches 9 10. Use this output to load 0000 2 into the counter with the ~LOAD line, pin 9. Since this is input is negated (that is what the ~ means) you will have to invert the output of your logic function before connecting it to pin 9. Use your oscilloscope as described previously to capture the LOAD (9) and clock pins (2). There should now be 10, not 16 clock pulses between roll-over events. Implement a 0-5 counter. The second counter should count the roll over events of the first counter which are no longer indicated by QD. Instead, wire ~LOAD of the first counter, which is your signal that forces it to roll over, to the CLK of the second counter. The second counter is now counting the number of times the first counter hit 9 and reset back to 0. So this is your second digit. Using the same procedure as previously described, design a single-gate logic function to detect when this counter hits 5 10, invert this output and wire it to ~LOAD, pin 9, of the second counter. The second counter should now count 0,1,2,3,4,5 and then roll over. Verify this with your oscilloscope by showing that there are 6 clock pulses input to the second counter (which are the LOAD signals to the first counter) for every roll-over event, as timed by the LOAD signal of the second counter. Put it all together. If you don t have the seven segment decoder and display already connected, reconnect them to your 0-5 and 0-9 counters. You now have a primitive clock that counts the oscillator input (CLK on counter 1) from 00 to 59, then rolls over. This could be the second or minute portion of a clock. You do not have to collect an particular data, but note in your lab report that you got the circuit to function. Version 1.1, 8/25/13 R. McLeod from earlier versions 5
Figure 5. A function generator driving a 74161 counter (top) which rolls over at 9. This in turn drives a second counter (bottom) which rolls over at 5. The entire circuit thus counts between 00 10 and 59 10 where each counter is a BCD representation of the two digits. Each BCD digit is decoded by a 7447 decoder into the seven lines required to control a 7-segment, common anode display. 5 EXTRA CREDIT SUGGESTION 2 (10 POINTS MAX) Read the datasheet on the 7447 decoder and find the names and functions of the other three pins on the chip. One at a time, disconnect one from Vcc and instead connect it to ground, which will activate this function since the lines are active low (that s what the ~ means). Document your results. Don t worry about RBO this is an output function that shares pin 4 with the input function ~BI. That is, test the function of the three inputs ~LT, ~RBI and ~BI. What could these be used for? NOTE: The RBI function does not work in multism this is a known bug. Version 1.1, 8/25/13 R. McLeod from earlier versions 6