Integrated Circuits (5.8) PDF - Aviation Australia

Integrated Circuits (5.8) Learning Objectives 5.8.1 Describe the operation and use of encoders and decoders (Level 2). 5.8.2 Describe the function of encoder types (Level 2). 5.8.3 Describe the uses of medium, large and very large scale integration devices (Level 2). 2024-11-05...

Integrated Circuits (5.8) Learning Objectives 5.8.1 Describe the operation and use of encoders and decoders (Level 2). 5.8.2 Describe the function of encoder types (Level 2). 5.8.3 Describe the uses of medium, large and very large scale integration devices (Level 2). 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 299 of 444 CASA Part 66 - Training Materials Only  Digital Integrated Circuits IC Format and Integration Scales Digital integrated circuits (ICs) are a collection of resistors, diodes and transistors fabricated on a single piece of semiconductor material (usually silicon) called a substrate, which is commonly referred to as a chip. The chip is enclosed in a protective plastic or ceramic package from which pins extend for connecting the IC to other devices. One of the more common types of packages is the Dual Inline Package (DIP), so called because it contains two parallel rows of pins. The pins are numbered anticlockwise when viewed from the top of the package with respect to an identifying notch or dot at one end. The DIP shown here is a 14-pin package that measures 0.75 in. by 0.25 in.; 16-, 20-, 24‑, 28-, 40- and 64-pin packages are also used. Aviation Australia IC Dual Inline Package (DIP) chip The silicon chip is connected to the pins of the DIP by very fine wires (1-mm diameter). The DIP is probably the most common digital IC package found in older digital equipment, but other types are becoming more and more popular. Digital ICs are often categorised according to their circuit complexity as measured by the number of equivalent logic gates on the substrate. Currently six levels of complexity are commonly defined. All of the specific ICs referred to in logic gates are SSI chips with a small number of gates. In modern digital systems, medium-scale integration (MSI) and large-scale integration devices (LSI, VLSI, ULSI, GSI) perform most of the functions that once required several circuit boards full of SSI devices. However, SSI chips are still used as the interface, or ‘glue’, between these more complex chips. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 300 of 444 CASA Part 66 - Training Materials Only  Complexity Gates per Chip Small-scale integration (SSI) Fewer than 12 Medium-scale integration (MSI) 12 - 99 Large-scale integration (LSI) 100 - 9 999 Very large-scale integration (VLSI) 10 000 - 99 999 Ultra large-scale integration (ULSI) 100 000 - 999 999 Giga-scale integration (GSI) 1 000 000 or more Some examples of MSI chips are encoders, decoders, multiplexers and de-multiplexers. LSI chips are used for the main memory in pocket calculators and computers from the 1970s. VLSI chips were used in the 1980s as CPU chips and RAM chips. Typically, small combinations of discrete gates are used to connect the larger ICs to each other or to external devices. Thus, it is necessary to know how to analyse, design, test and troubleshoot simple combinational circuits. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 301 of 444 CASA Part 66 - Training Materials Only  Decoder A decoder is a logic circuit that accepts a set of inputs representing a binary number and activates only the output that corresponds to that input number. In other words, a decoder circuit looks at its inputs, determines which binary number is present there and activates the one output that corresponds to that number; all other outputs remain inactive. The diagram for a general decoder is shown with N inputs and M outputs. Since each of the N inputs can be 0 or 1, there are 2N possible input combinations or codes. For each of these input combinations, only one of the M outputs will be active (HIGH); all the other outputs are LOW. Many decoders are designed to produce active LOW outputs where only the selected output is LOW while all others are HIGH. This can be indicated by the presence of small circles on the output lines in the decoder diagram. Aviation Australia Decoder block diagram Some decoders do not utilise all of the 2N possible input codes, but only certain ones. For example, a BCD-to-decimal decoder has a 4-bit input code and 10 output lines that correspond to the 10 BCD code groups 0000 through 1001. Decoders of this type are often designed so that if any of the unused codes are applied to the input, none of the outputs will be activated. The illustration shows the circuit for a decoder with three inputs and 23 = 8 outputs. It uses all AND gates, so the outputs are active HIGH. Note that for a given input code, the only output that is active HIGH is the one corresponding to the decimal equivalent of the binary input code (e.g. output O6 goes HIGH only when CBA = 1102 = 610). 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 302 of 444 CASA Part 66 - Training Materials Only  Aviation Australia 3 line to 8 line decoder 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 303 of 444 CASA Part 66 - Training Materials Only  A BCD to Seven-Segment Decoder/Driver A BCD to seven-segment decoder is used to take a 4-bit BCD input and provide the outputs that will pass current through the appropriate segments to display the decimal digit. The logic for this decoder is more complicated than that of decoders we have looked at previously because each output is activated for more than one combination of inputs. For example, the 'e' segment must be activated for any of the digits 0, 2, 6 and 8, which means whenever any of the codes 0000, 0010, 0110 or 1000 occurs. The figure shows a BCD to seven-segment decoder/driver (TTL 7446 or 7447) being used to drive a seven-segment LED readout. Each segment consists of one or two LEDs. The anodes of the LEDs are all tied to Vcc (+ 5 V). The cathodes of the LEDs are connected through current limiting resistors to the appropriate outputs of the decoder/driver. The decoder/driver has active LOW outputs, which are open collector driver transistors that can sink a fairly large current. This is because LED readouts may require 10 to 40 mA per segment, depending on their type and size. To illustrate the operation of this circuit, let us suppose that the BCD input is D = 0, C = 1, B = 0, A = 1, which is BCD for 5. With these inputs, the decoder driver outputs a, f, g, c and d will be driven LOW (connected to ground), allowing current to flow through the a, f, g, c and d LED segments and thereby displaying the numeral 5. The b and e outputs will be HIGH (open) so that LED segments b and e cannot conduct. The 7446/47 decoder/drivers are designed to activate specific segments even for non-BCD input codes (greater than 1001). The lower part of the illustration shows the activated segment patterns for all possible input codes from 0000 to 1111. Note that an input code of 1111 (15) will blank out all the segments. Aviation Australia BCD to seven-segment decoder driver 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 304 of 444 CASA Part 66 - Training Materials Only  Seven-segment decoder/drivers such as the 7446/47 are exceptions to the rule that decoder circuits activate only one output for each combination of inputs. Rather, they activate a unique pattern of outputs for each combination of inputs. Encoders Most decoders accept an input code and produce a HIGH (or a LOW) at one and only one output line. In other words, we can say that a decoder identifies, recognises or detects a particular code. The opposite of this decoding process is called encoding and is performed by a logic circuit called an encoder. An encoder has a number of input lines, only one of which is activated at a given time, and produces an N bit output code, depending on which input is activated. The illustration shown is the general diagram for an encoder with M inputs and N outputs. Here the inputs are active HIGH, which means they are normally LOW. Aviation Australia Encoder block diagram 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 305 of 444 CASA Part 66 - Training Materials Only  Encoder 8 to 3 We saw that a binary to octal decoder (3-line to 8-line decoder) accepts a 3-bit input code and activates one of eight output lines corresponding to that code. An octal to binary encoder (8-line to 3- line encoder) performs the opposite function: it accepts eight input lines and produces a 3-bit output code corresponding to the activated input. The diagram shows the logic circuit and the truth table for an octal-to-binary encoder with active LOW inputs. By following through the logic, you can verify that a LOW at any single input will produce the output binary code corresponding to that input. For instance, a LOW at A3 (while all other inputs are HIGH) will produce O2 = 0, O1 = 1, O0=1 which is the binary code for 3. Notice that A0 is not connected to the logic gates because the encoder outputs will normally be at 000 when none of the A1 to A9 inputs is LOW. Aviation Australia Encoder circuit and logic diagram 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 306 of 444 CASA Part 66 - Training Materials Only  Priority Encoder A priority encoder includes the necessary logic to ensure that when two or more inputs are activated, the output code corresponds to the highest numbered input. For example, when both A3 and A5 are LOW, the output code will be 101 (5). Similarly, if A6 and A4 (or any input from A1 to A5) and A0 were all LOW, the output code would remain 110 (6). 74147 Decimal-to-BCD Priority Encoder The illustration shows the logic symbol and the truth table for the 74147 (74LS147, 74HC147), which functions as a decimal-to-BCD priority encoder. It has nine active LOW inputs representing the decimal digits 1 through 9, and it produces the inverted BCD code corresponding to the highest- numbered activated input. Aviation Australia Decimal to BCD encoder block diagram and logic diagram Let us examine the truth table to see how this IC works. The first line in the table shows all inputs in their inactive HIGH state. For this condition, the outputs are 1111, which is the inverse of 0000, the BCD code for 0. The second line in the table indicates that a LOW at A9, regardless of the states of time for other inputs, will produce an output code of 0110, which is the inverse of 1001, the BCD code for 9. The third line shows that a LOW at A8, provided that A9 is HIGH, will produce an output code of 0111, the inverse of 1000, the BCD code for 8. In a similar manner, the remaining lines in the table show that a LOW at any input, provided that all higher-numbered inputs are HIGH, will produce the inverse of the BCD code for that input. The 74147 outputs will normally be HIGH when none of the inputs are activated. This corresponds to the decimal 0 input condition. There is no A0 input since the encoder assumes the decimal 0 input state when all other inputs are HIGH. The 74147 inverted BCD outputs can be converted to normal BCD by putting each one through an inverter. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 307 of 444 CASA Part 66 - Training Materials Only  Switch Encoder The diagram shows how a 74147 can be used as a switch encoder. The 10 switches might be the keyboard switches on a calculator representing digits 0 through 9. The switches are of the normally open type so that the encoder inputs are all normally HIGH and the BCD output is 0000 (note the inverters). When a digit key is depressed, the circuit will produce the BCD code for that digit. Since the 74LS147 is a priority encoder, simultaneous key depressions will produce the BCD code for the higher- numbered key. Aviation Australia Decimal to BCD encoder circuit The switch encoder shown in the illustration can be used whenever BCD data must be manually entered into a digital system. A prime example is in an electronic calculator, where the operator depresses several keyboard switches in succession to enter a decimal number. In a basic calculator, the BCD code for each decimal digit is entered into a 4-bit storage register. In other words, when the first key is depressed, the BCD code for that digit is sent to a 4-bit FF register; when the second switch is depressed, the BCD code for that digit is sent to another 4-bit FF register and so on. Thus, a calculator that can handle eight digits will have eight 4-bit registers to store the BCD codes for these digits. Each 4-bit register drives a decoder/driver and a numerical display so that the eight-digit number can be displayed. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 308 of 444 CASA Part 66 - Training Materials Only  Excess-3 Encoding Excess-3 encoding is a self-complementary binary-coded decimal code and numeral system. It is a biased representation. Biased codes are a way to represent values with a balanced number of positive and negative numbers using a pre-specified number N as a biasing value. In Excess-3, numbers are represented as decimal digits, and each digit is represented by 4 bits as the digit value plus 3 (the ‘excess’ amount). Aviation Australia Decimal to BCD and excess-3 conversion table 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 309 of 444 CASA Part 66 - Training Materials Only  Gray Encoding Gray encoding is a binary numeral system in which two successive values differ in only 1 bit (binary digit). The reflected binary code was originally designed to prevent spurious output from electromechanical switches. Gray encoding was often used to encode altimeters of the non-servo type. In this system, the bellows drives a glass disc etched with transparent and opaque sectors. A light source shines through the disc onto photoelectric cells, which convert the disc’s movement into coded signals for the transponder. This type of pick-off provides a high degree of accuracy with very low torque requirements. Aviation Australia Gray code used for altimeter encoding 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 310 of 444 CASA Part 66 - Training Materials Only