Interestingly, questions on TTL and a couple of questions on logic symbols were removed from this section to make way for questions on digital comparators and programmable logic devices….Dan
E6C – Digital ICs: Families of digital ICs; gates; Programmable Logic Devices (PLDs)
Integrated circuits (ICs) are now an integral part (pun intended) of amateur radio electronics. There are several different technologies used to manufacture IC including transistor-transistor logic, or TTL; complementary metal-oxide semiconductor, or CMOS; and BiCMOS, which uses a combination of bipolar and CMOS transistors.
CMOS is arguably the most common type of digital IC. An advantage of CMOS logic devices over TTL devices is that the have lower power consumption. (E6C05) CMOS digital integrated circuits also have high immunity to noise on the input signal or power supply because the input switching threshold is about one-half the power supply voltage. (E6C06)
BiCMOS logic is an integrated circuit logic family using both bipolar and CMOS transistors. (E6C12) An advantage of BiCMOS logic is that it has the high input impedance of CMOS and the low output impedance of bipolar transistors. (E6C13)
Tri-state logic devices are logic devices with 0, 1, and high impedance output states. (E6C03) These devices can be made with either TTL or CMOS technology. The primary advantage of tri-state logic is the ability to connect many device outputs to a common bus. (EC604) When a device’s outputs are in the high-impedance state, they act as if they are disconnected.
When working with digital ICs, it is important to recognize the various symbols for the different types of logic gates. In Figure E6-5, 2 is the schematic symbol for a NAND gate. (E6C08) In Figure E6-5, 4 is the schematic symbol for a NOR gate. (E6C10) In Figure E6-5, 5 is the schematic symbol for the NOT operation (inverter). (E6C11)
When designing circuits with digital ICs, you may not use all of the inputs of the gates in that IC. To set that input to a digital 1 or 0, you might use a pull-up resistor or a pull-down resistor. A pull-up or pull-down resistor is best described as a resistor connected to the positive or negative supply line used to establish a voltage when an input or output is an open circuit. (E6C07) Pull-up resistors are also often used on the output of a digital circuit to prevent that output from floating.
A particular type of IC is called a comparator. Comparators compare an input voltage to a threshold voltage, and when the level of a comparator’s input signal crosses the threshold is the comparator changes its output state. (E6C02)
Comparators have a property called hysteresis. Basically, what this amounts to is that the threshold voltage at which the comparator switches is lower when the input voltage is decreasing than the threshold voltage at which the comparator switches when the input voltage is increasing. The function of hysteresis in a comparator is to prevent input noise from causing unstable output signals. (E6C01) If the threshold voltage was the same for both increasing and decreasing input voltages, and the input voltage was right at the threshold voltage, then noise could cause that input voltage to go above and below the threshold randomly. If the comparator input did not have hysteresis, then its output would switch randomly.
Many modern electronic devices now use programmable logic devices instead of cobbling together a digital circuit with a collection of ICs with simple gates. A Programmable Logic Device (PLD) is a programmable collection of logic gates and circuits in a single integrated circuit. (E6C09). Programmable logic devices can have thousands or even millions of gates in a single IC. To design digital circuits with PLDs, designers use computer-aided design software to connect and configure the logic gates.
A programmable gate array is a particular type of programmable logic device. The primary advantage of using a Programmable Gate Array (PGA) in a logic circuit is that complex logic functions can be created in a single integrated circuit. (E6C14)