Digital Systems Laboratory - Eskişehir Osmangazi Üniversitesi

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. For only the output “a”, obtain a minimum logic function. Realize this function using NAND gates andinverters only. For example if decimal 9 is to be displayed a, b, c, d, f, g must be 0 and the others must be1 (For common anode type display units), if decimal 5 is to be displayed then a, f, g, c, d must be 0 andthe others must be 1.Procedure:1. Gray code to Binary converter:Gray code is one of the codes used in digital systems. It has the advantage over binary numbers that onlyone bit in the code word changes when going from one number to the next. (See Table 2). Construct thecircuit (PreLab Q.1) and verify its operations.Table 6Fig. 15 XOR chip 7486Decimal Gray Binary0 0000 00001 0001 00012 0011 00103 0010 00114 0110 01005 0111 01016 0101 01107 0100 01118 1100 10009 1101 100110 1111 101011 1110 101112 1010 110013 1011 110114 1001 111015 1000 1111.2. BCD-to-seven Segment converter:A light emitting Diode (LED) is a PN junction diode. When the diode is forward biased, a current flowsthrough the junction and the light is emitted. See Fig.16.Fig. 16A seven segment LED display contains 7 LEDs. Each LED is called a segment and they are identified as(a, b, c, d, e, f, g) segments Fig.17.18
Fig. 17 Decimal digits represented by the 7 segmentsThe display has 7 inputs each connected to an LED segment. All anodes of LEDs are tied together andjoined to 5 volts (this type is called common anode type). A current limiting resistance network must beused at the inputs to protect the 7-segment from overloading. BCD inputs are converted into 7 segmentinputs (a, b, c, d, e, f, g) by using a decoder, as shown in Fig.18.Fig. 18A decoder is a combinational circuit that converts binary information from n input lines to a maximum of 2n output lines. The input to the decoder is a BCD code and the outputs of the systems are the sevensegments a, b, c, d, e, f, and g. For further information and pin connections, consult the specificationsheet for decoder and 7-segment units.The 7447 BCD-to-seven segment decoder is used to drive a seven-segment LED display. The outputs, a-g, drive the corresponding segments on the seven-segment display according to the binary numberpresent at the inputs A-D, D being the most significant bit of the number. Three additional inputs, LAMPTEST, BI/RBO, and RBI are provided. The blanking input/ripple-blanking output (BI/RBO) blanks (turnsoff) the display when set LOW. Otherwise, when BI/RBO is set high, the outputs drive the displayaccording to the inputs A-D. The ripple blanking input (RBI) must be HIGH if blanking of a decimal zero isnot desired. This input is useful in blanking higher order zeroes when using several displays for a multidigitdecimal number. Finally, LAMP TEST selects (turns on) all the segments when set LOW. It is used totest the segments on the display.a. Construct the combinational circuit (PreLab Q.2) which will simulate the decoder function for only thesegment “a”, of the display.b. Connect the output “a” of your circuit to appropriate input of 7-segment display unit through a currentlimiting resistor, 330/470 Ω. By applying BCD codes verify the displayed decimal digits for thatsegment for “a” of the display.c. Replace your circuit by a decoder IC 7447 for all of the seven segments. Use current limiting resistorsat the inputs of the seven-segment display. Observe the display and record the segments that willlight up for invalid inputs sequence.d. Comment on the design if you don’t want to see any digit for invalid input sequence.19
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