What an integrated circuit is
An integrated circuit (IC) is a complete circuit — anything from a few transistors to billions — formed together on a single small chip of silicon, the die. Transistors, diodes, resistors and small capacitors are made in and on the chip by repeated doping, oxidation and metal-deposition steps, and interconnected by metal tracks on its surface. Fine gold or aluminium bond wires join the die to the package's lead frame, and the whole thing is sealed in plastic or ceramic. Because everything is made in one piece, this is called a monolithic IC. Hybrid circuits mount several chips and printed resistors on a ceramic substrate.
- Advantages: tiny size and weight; very high reliability, because thousands of connections are made inside the chip instead of by solder joints; low power; low cost per function; high speed.
- Limitations: little power-handling ability; inductors and large capacitors cannot be integrated; a faulty IC cannot be repaired, so the chip, card or LRU is replaced; and many are sensitive to static and heat.
- Two big families: digital (logic) ICs, which handle signals that are either HIGH or LOW, and linear (analogue) ICs, which handle continuously varying signals — operational amplifiers, regulators, audio and sensor amplifiers (Chapter 6). Mixed-signal chips such as converters and timers do both.
- Scale of integration (roughly): SSI up to about 10 gates, MSI about 10–100, LSI about 100–10 000, VLSI beyond that — modern processors hold billions of transistors.
Packages and pin numbering
- DIP (dual in-line package): two rows of pins on a 2.54 mm (0.1 in) pitch, for through-hole mounting or a socket. Pin 1 is identified by a notch at one end or a dot beside it. Seen from above with the notch at the top, pins are numbered anticlockwise — down the left side from pin 1 and back up the right. On a 14-pin DIP, pin 14 is therefore opposite pin 1.
- Surface-mount packages: SOIC (small-outline, gull-wing leads on two sides), PLCC (J-leads on four sides), QFP (quad flat pack, fine gull-wing leads on four sides) and BGA (ball grid array, solder balls under the body, so the joints cannot be seen and are checked by X-ray). The same rule applies: find the pin-1 mark, then count anticlockwise viewed from the top.
- Socketed ICs must go back in the same way round, matching the notch to the socket's mark. Never insert or remove them with power on.
Logic levels
Digital circuits work with two states: 1 and 0, also called HIGH/LOW, TRUE/FALSE or ON/OFF. Each logic family defines a voltage band for each state, with a forbidden band between them.
- TTL (transistor–transistor logic, the 74-series): a 5 V supply. An input below 0.8 V is LOW and above 2.0 V is HIGH; outputs are guaranteed below 0.4 V or above 2.4 V, leaving a noise margin.
- CMOS (4000 series at 3–18 V, 74HC at 2–6 V and today's 3.3 V and 1.8 V parts): its thresholds are roughly 30% and 70% of the supply. It uses very little power. Unused CMOS inputs must be tied HIGH or LOW — a floating input picks up noise and can make the chip oscillate or overheat.
- Positive logic: HIGH = 1 — the normal convention. Negative logic: LOW = 1. Signals that are "true when LOW" (active-low) are marked by a bubble on the gate or a bar over the signal name.
Aircraft systems pass many simple on/off signals between boxes as discretes, typically 28 V/open or ground/open. Interface circuits inside each LRU convert them to the chip's own logic levels.
The basic gates
A logic gate produces an output that depends only on its present inputs. A truth table lists the output for every input combination — 2ⁿ rows for n inputs, so 4 rows for two inputs and 8 for three. Boolean algebra writes the functions as expressions.
| Gate | Expression | Output is 1 when… |
|---|---|---|
| NOT (inverter) | Y = A | the input is 0 — it always gives the opposite |
| Buffer | Y = A | the input is 1 — same logic, but more drive current or isolation |
| AND | Y = A·B | all inputs are 1 |
| OR | Y = A + B | any input is 1 |
| NAND | Y = A·B | not all inputs are 1 — it is 0 only when all inputs are 1 |
| NOR | Y = A + B | no input is 1 — it is 1 only when all inputs are 0 |
| XOR (exclusive OR) | Y = A ⊕ B | the inputs are different |
| XNOR (exclusive NOR) | Y = A ⊕ B | the inputs are the same — an equality (coincidence) detector |
NAND and NOR are universal: any logic function can be built from NAND gates alone, or from NOR gates alone. Tie a NAND's inputs together and you have a NOT. De Morgan's theorem links the two families: A·B = A + B and A + B = A·B. So a NAND is an OR with inverted inputs, and a NOR is an AND with inverted inputs. It also means a gate's job depends on the logic convention: in negative logic, a positive-logic AND gate acts as an OR, and a NAND as a NOR.
ANSI and IEC symbols
Two symbol standards are in everyday use, and both appear in maintenance manuals and in exams. The ANSI/IEEE distinctive shapes are common in American documentation. The IEC 60617 rectangular symbols are common in European documentation; they write the function inside the box.
Logic in aircraft systems
Warning, interlock and indication systems are logic. Examples are the gear and take-off configuration warnings, air/ground (weight-on-wheels) logic, reverser deploy interlocks (on the ground and lever selected) and BITE. Older aircraft built them from discrete gate ICs and relays; modern aircraft do the same thing in software or programmable logic inside an LRU. The maintenance manual still documents it as a logic diagram drawn with gate symbols, and fault isolation means tracing which input is stopping the output.
- Gates whose output depends only on the present inputs make up combinational logic. Circuits with memory — flip-flops, counters, registers — are sequential logic, covered with number systems and computers in Module 5.
- Logic ICs, especially CMOS, are static sensitive (Chapter 7). Handle cards by their edges, use a wrist strap, and keep spares in their shielding bags.