What do they do?
| CMOS integrated circuits are widely used as Process units. |
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How do they operate?
Most of the ‘Process’ units covered in these data sheets are Complementary Metal Oxide Semiconductor (CMOS) 4000B series integrated circuits. The only exceptions are PICs, 555 timers and operational amplifiers. This data sheet gives general information about this important family of ICs.
CMOS ICs are designed so that they all share a set of common features, which are listed below. This makes it easy to connect CMOS ICs to each other.
CMOS ICs are made up of a large number of MOSFETs, all on a single piece of silicon.
Common features of all CMOS ICs
| Power supply voltage (Vs) |
Any d.c. voltage between 3 and 15V. |
| Digital ‘high’ or ‘1’ voltage |
At least 0.7 ´ Vs. So, for a supply voltage of 5V, the input signal voltage must be more than 3.5V to give a digital ‘high’. |
| Digital ‘low’ or ‘0’ voltage |
Less than 0.3 ´ Vs. So, for a supply voltage of 5V, the input signal voltage must be less than 1.5V to give a digital ‘low’. |
| Output signal current |
Up to 0.12mA is guaranteed if Vs = 5V. In practice, CMOS ICs can usually provide more current (and many published circuit diagrams make use of this) but it is bad practice to design circuits which exceed the manufacturer’s specification. |
| Input signal current |
Up to 1mA. |
| Operating temperature |
Between -40oC and +85oC |
Power supply
On nearly all CMOS ICs, if pin 1 (information on IC pins) is at the ‘top left hand corner’, then the negative side of the power supply (0V) is connected to the ‘bottom left-hand corner’ pin. On manufacturers’ data sheets this pin is labelled VSS (standing for the ‘Source’ voltage).
The positive side of the power supply (+Vs) is usually connected to the ‘top right-hand corner’ pin (labelled VDD – ‘Drain’ voltage – on manufacturers’ data sheets).
Using ‘spare’ gates
Most CMOS ICs used for gates contain several gates. For example, the 4011B CMOS IC contains four two-input NAND gates
If a circuit needs, for example, one AND gate, one NAND gate and one inverter it is possible to produce all three of these gates using just one 4011B IC. This saves money and makes the PCB simpler and more reliable.
Pins of 4011B NAND IC
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To make an inverter out of a NAND (or NOR) gate, we simply connect the two input pins together.
From the truth table for the NAND gate:
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We can see that if both input pins are connected together (so that both input signals are the same) then:
- if the input signal is ‘0’ then the output signal will be ‘1’;
- if the input signal is ‘1’ then the output signal will be ‘0’.
This is exactly what we would expect from an inverter.
In a similar way, from the truth table for a NOR gate, if both its input pins are connected it will also act as an inverter. |
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If we compare the truth table for a NAND gate:
and the truth table for an AND gate:
it is clear that the output signal from an AND gate is the inverse of the signal from a NAND gate. So, if we follow a NAND gate with an inverter, the two in combination will act as an AND gate. |
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So, if we use a NAND gate, followed by a second NAND gate with its input pins connected (so that it works like an inverter) we have made an AND gate out of two NAND gates.
In just the same way, a NOR gate, followed by a second NOR gate with its input pins connected will work as an OR gate.
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So, the circuit on the left will act as:
- an AND gate – with input signals A and B and output signal X
- a NAND gate – with input signals C and D and output signal Y, and
- an inverter – with input signal E and output signal Z
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The PCB on the left is based on the above circuit. As can be seen, by using these techniques, the AND, NAND and inverter can all be produced with just one 4011B IC.
The PCB also illustrates that, if this approach is used, it is difficult to avoid using links, unless the tracks are run between IC pins.
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Some common CMOS ICs
The pin connections of the CMOS ICs mentioned in these data sheets are shown below.
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4001 NOR Gates
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4011 NAND Gates
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4026 Counter
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4029 Counter
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4069 Inverters
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Pins of 4070B
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4071 NOR Gates
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4081 AND Gates
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40106B Schmitt Inverter
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Making
CMOS ICs should not be stored in plastic containers (unless the containers are suitable for electronic components) since this can generate excessive static which can damage them. For the same reason, when designing the PCB, any unused input pins should be connected to 0V or Vs.
A dual in line (DIL) socket should be used with all ICs. This makes testing and repair much easier.
Before inserting the IC, connect the power supply and use a voltmeter to check that:
- the voltage on pin 7 (for most CMOS ICs) is low (0V);
- the voltage on pin 14 (for most CMOS ICs) is high (the supply voltage).
Insert the IC the right way round.
Alternatives
- PICs - more flexible but more expensive
Web links
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