FLIP FLOP EXCITATION TABLE

The excitation table is similar to the characteristic table that we discussed on flip-flops. The excitation table lists the present state, the desired next state and the flip-flop inputs (J, K, D, etc.) required to achieve that.

The same for a J-K flip-flop and a D flip-flop are shown in Tables 11.7 and 11.8 respectively. Referring to Table 11.7, if the output is in the logic ‘0’ state and it is desired that it goes to the logic ‘1’ state on occurrence of the clock pulse, the J input must be in the logic ‘1’ state and the K input can be either in the logic ‘0’ or logic ‘1’ state.

This is true as, for a ‘0’ to ‘1’ transition, there are two possible input conditions that can achieve this. These are J = 1, K = 0 (SET mode) and J = K = 1 (toggle mode), which further leads to J = 1# K = X (either 0 or 1). The other entries of the excitation table can be explained on similar lines.

In the case of a D flip-flop, the D input is the same as the logic status of the desired next state. This is true as, in the case of a D flip-flop, the D input is transferred to the output on the occurrence of the clock pulse, irrespective of the present logic status of the Q output.

TYPES OF LOGIC FAMILY BASIC AND TUTORIALS

The entire range of digital ICs is fabricated using either bipolar devices or MOS devices or a combination of the two. Different logic families falling in the first category are called bipolar families, and these include diode logic (DL), resistor transistor logic (RTL), diode transistor logic (DTL), transistor transistor logic (TTL), emitter coupled logic (ECL), also known as current mode logic (CML), and integrated injection logic (I2L).

The logic families that use MOS devices as their basis are known as MOS families, and the prominent members belonging to this category are the PMOS family (using P-channel MOSFETs), the NMOS family (using N-channel MOSFETs) and the CMOS family (using both N- and P-channel devices).

The Bi-MOS logic family uses both bipolar and MOS devices. Of all the logic families listed above, the first three, that is, diode logic (DL), resistor transistor logic (RTL) and diode transistor logic (DTL), are of historical importance only.

Diode logic used diodes and resistors and in fact was never implemented in integrated circuits. The RTL family used resistors and bipolar transistors, while the DTL family used resistors, diodes and bipolar transistors.

Both RTL and DTL suffered from large propagation delay owing to the need for the transistor base charge to leak out if the transistor were to switch from conducting to nonconducting state. Figure 5.1 shows the simplified schematics of a two-input AND gate using DL [Fig. 5.1(a)], a two-input NOR gate using RTL [Fig. 5.1(b)] and a two-input NAND gate using DTL [Fig. 5.1(c)]. The DL, RTL and DTL families, however, were rendered obsolete very shortly after their introduction in the early 1960s owing to the arrival on the scene of transistor transistor logic (TTL).

Logic families that are still in widespread use include TTL, CMOS, ECL, NMOS and Bi-CMOS. The PMOS and I2L logic families, which were mainly intended for use in custom large-scale integrated (LSI) circuit devices, have also been rendered more or less obsolete, with the NMOS logic family replacing them for LSI and VLSI applications.

COMMONLY USED IC COUNTERS AND REGISTERS BELONGING TO TTL CMOS & ECL LOGIC FAMILIES

A Table of Commonly used IC counters and registers belonging to the TTL, CMOS and ECL logic families


Type Number Function Logic FAMILY

7490 Decade counter TTL
7491 Eight-bit shift register (serial-in/serial-out) TTL
7493 Four-bit binary counter TTL
74160 BCD decade counter with asynchronous CLEAR TTL
74161 Four-bit binary counter with asynchronous CLEAR TTL
74162 BCD decade counter with synchronous CLEAR TTL
74163 Four-bit binary counter with synchronous CLEAR TTL

74164 Eight-bit shift register (serial-in/parallel-out) TTL
74165 Eight-bit shift register (parallel-in/serial-out)
74166 Eight-bit shift register (parallel-in/serial-out) TTL
74178 Four-bit parallel access shift register TTL
74190 Presettable BCD decade UP/DOWN counter TTL
74191 Presettable four-bit binary UP/DOWN counter TTL
74192 Presettable BCD decade UP/DOWN counter TTL
74193 Presettable four-bit binary UP/DOWN counter TTL
74194 Four-bit right/left universal shift register TTL
74198 Eight-bit universal shift register (parallel-in/parallel-out bidirectional) TTL
74199 Eight-bit universal shift register (parallel-in/parallel-out bidirectional) TTL
74290 Decade counter TTL
74293 Four-bit binary counter TTL
74390 Dual decade counter TTL
74393 Dual four-bit binary counter TTL
4014 B Eight-bit static shift register CMOS
(synchronous parallel or serial-in/serial-out)
4015 B Dual four-bit static shift register CMOS
(serial-in/parallel-out)
4017 B Five-stage Johnson counter CMOS
4021 B Eght-bit static shift register CMOS
(asynchronous parallel-in or synchronous serial-in/serial-out)
4029 B Synchronous presettable four-bit UP/DOWN counter CMOS
4035 B Four-bit universal shift register CMOS
40160 B Decade counter with asynchronous CLEAR CMOS
40161 B Binary counter with asynchronous CLEAR CMOS
40162 B Decade counter CMOS
40163 B Binary Counter CMOS
40192 B Presettable BCD UP/DOWN counter CMOS
40193 B Presettable Binary UP/DOWN counter CMOS
4510 B Presettable UP/DOWN BCD counter CMOS
4518 B Dual four-bit decade counter CMOS
4520B Dual four-bit binary counter CMOS
4522 B Four-bit BCD programmable divide-by-N counter CMOS
4722 B Programmable counter/timer CMOS
4731 B Quad 64-bit static shift register CMOS
MC 10136 Universal hexadecimal counter ECL
MC 10137 Universal decade counter ECL
MC 10141 Four-bit universal shift register ECL
MC 10154 Binary counter (four-bit) ECL
MC 10178 Four-bit binary counter ECL

THE 7400-SERIES DISCRETE LOGIC FAMILY IC BASICS AND TUTORIALS

WHAT IS THE 7400-SERIES DISCRETE LOGIC FAMILY INTEGRATED CIRCUITS?


With the advent of ICs in the early 1960s, engineers needed ready access to a library of basic logic gates so that these gates could be wired together on circuit boards and turned into useful products. Rather than having to design a custom microchip for each new project, semiconductor companies began to recognize a market for standard, off-the-shelf logic ICs.


In 1963 and 1964, Sylvania and Texas Instruments began shipment of the 7400-series discrete logic family and unknowingly started a de facto industry standard that lasts to this day and shows no signs of disappearing anytime soon.

Using the 7400 family, an engineer can select logic gates, flip-flops, counters, and buffers in individual packages and wire them together as desired to solve a specific problem. Some of the most common members of the 7400 family are listed in Table 2.1.

TABLE 2.1 Common 7400 ICs
Part Number Function Number of Pins
7400 Quad two-input NAND gates 14
7402 Quad two-input NOR gates 14
7404 Hex inverters 14
7408 Quad two-input AND gates 14
7432 Quad two-input OR gates 14
7447 BCD to seven-segment display decoder/driver 16
7474 Dual D-type positive edge triggered flip-flops 14
7490 Four-bit decade counter 14
74138 Three-to-eight decoder 16
74153 Dual 4-to-1 multiplexer 16
74157 Quad 2-to-1 multiplexers 16
74160 Four-bit binary synchronous counter 16
74164 Eight-bit parallel out serial shift registers 16
74174 Quad D-type flip-flops with complementary outputs 16
74193 Four-bit synchronous up/down binary counter 16
74245 Octal bus transceivers with tri-state outputs 20
74373 Octal D-type transparent latch 20
74374 Octal D-type flip-flops 20

These are just a few of the full set of 7400 family members. Many 7400 parts are no longer used, because their specific function is rarely required as a separate chip in modern digital electronics designs.

However, the parts listed above, and many others that are not listed, are still readily available today and are commonly found in a broad range of digital designs ranging from low-end to hightech devices. 7400-series logic has been available in DIPs for a long time, as well as (more recently) SOICs and other high-density surface mount packages.

All flavors of basic logic gates are available with varying numbers of inputs. For example, there are 2-, 3-, and 4-input AND gates and 2-, 3-, 4-,8-, 12-, and 13-input NAND gates. There are numerous varieties of flip-flops, counters, multiplexers, shift registers, and bus transceivers.

Flip-flops exist with and without complementary outputs, preset/ clear inputs, and independent clocks. Counters are available in 4-bit blocks that can both increment and decrement and count to either 15 (binary counter) or 9 (decade counter) before restarting the count at 0.

Shift registers exist in all permutations of serial and parallel inputs and outputs. Bus transceivers in 4- and 8 bit increments exist with different types of output enables and capabilities to function in unidirectional or bidirectional modes. Bus transceivers enable the creation and expansion of tri-state buses on which multiple devices can communicate.

One interesting IC is the 7447 seven-segment display driver. This component allows the creation of graphical numeric displays in applications such as counters and timers. Seven-segment displays are commonly seen in automobiles, microwave ovens, watches, and consumer electronics.

Seven independent on/off elements can represent all ten digits.. The 7447 is able to drive an LED-based seven-segment display when given a binary coded decimal (BCD) input. BCD is a four-bit binary number that has valid values from 0 through 9.

Hexadecimal values from 0xA through 0xF are not considered legal BCD values. Familiarity with the 7400 series proves very useful no matter what type of digital system you are designing.

For low-end systems, 7400-series logic may be the only type of IC at your disposal to solve a wide range of problems. At the high end, many people are often surprised to see a small 14- pin 7400-series IC soldered to a circuit board alongside a fancy 32-bit microprocessor running at 100 MHz.

The fact is that the basic logic functions that the 7400 series offers are staples that have direct applications at all levels of digital systems design. It is time well spent to become familiar with the extensive capabilities of the simple yet powerful 7400 family.

Manufacturers’ logic data books, either in print or on line, are invaluable references. It can be difficult to know ahead of time if a design may call for one more gate to function properly; that is when a 40-year old logic family can save the day.