Background for Digital Design
3.5 TRANSMISSION GATES AND TRI-STATE DRIVERS
--(a) (c) (d)
fiGURE 3.6
The inverter, its I/O behavior, and its two logic interpretations. (a) The CMOS transistor circuit.
(b) Physical truth table. (c) high-to-active-Iow conversion and logic circuit symbol. (d) Active-low-to-active-high conversion and logic circuit symbol.
the physical truth table and logic interpretations given in Figs. 3.6b, 3.6c, and 3.6d. More detailed information on these logic families is presented in Appendix A.
3.5 TRANSMISSION GATES AND TRI-STATE DRIVERS
A MOS transistor switch that functions as a passive (non-amplifying) switching device and that does not invert a voltage signal is called a transmission gate or pass transistor.
Logic circuits composed of transmission gates are called steering logic and are discussed in Section 6.9. Shown in Fig. 3.7 are the circuit symbols and equivalent circuits for the NMOS, PMOS, and CMOS transmission gates. Here, it can be seen that the ON condition in Fig. 3.7b permits an input signal Xi to be transferred to the output; hence, X 0 Xi. Conversely, the OFF condition disconnects the output from the input, allowing no signal to be transferred.
Notice that the CMOS transmission gate requires complementary "enable" inputs, EN and to the NMOS and PMOS gates, respectively. This simply means that when one enable input is at high voltage (HV) the other must be at low voltage (LV) and vice versa.
As indicated earlier, an NMOS switch passes LV well but not HV, the reverse being true for a PMOS switch. Consequently, some distortion of the transmitted waveform is to be expected in NMOS and PMOS transmission gates operated in the transfer mode (ON condition). The CMOS switch, on the other hand, combines the best of both worlds, thereby minimizing waveform distortion.
An active (restoring) switching device that operates in either a transfer or disconnect mode is called a tri-state driver or tri-state buffer. If in the transfer mode it is designed to invert, it is called an inverting tri-state driver. These devices are called "tri-state" or "three-state" because they operate in one of three states -logic 0, logic 1, or high-impedance (Hi-Z) state. In the Hi-Z or disconnect state the tri-state driver is functionally "floating;' as if it were not there. Tri-state drivers are used to interface various IC devices to a common
3.5 TRANSMISSION GATES AND TRI-STATE DRIVERS
Transmission gate circuit symbols and their idealized equivalent circuits. (a) Simplified circuit symbols for NMOS, PMOS, and CMOS transmission gates. (b) ON (transfer) mode equivalent circuit.
(c) OFF (disconnect) mode equivalent circuit.
data bus so that the devices will not interfere with each other. By this means, tri-state drivers permit multiple signal sources to share a single line if only one of the signals is active at any given time. These drivers also serve as a controlled enable on the output of some devices.
Note that the term "tri-state" is a trademark of National Semiconductor Corporation. Thus, the use ofthe term "tri-state" in this text acknowledges NSC's right of trademark. The terms tri-state and three-state are often used interchangeably.
Shown in Fig. 3.8 are four types of CMOS tri-state drivers constructed from the equivalent of two or three inverters. They differ in the activation levels of the control input, C, and the output, X". indicated by the logic circuit symbols. The choices are inverting or noninverting tri-state drivers with active high or active low control inputs, as provided in Figs. 3.8a-d. The buffering (driving) strength is the same for all tri-state drivers, however. This is so because during the transfer stage the outputs Xo are connected to the supply
+
VDD depending on the character of the driver and the Xi voltage level. For example, in the case of the inverting tri-state driver of Fig. 3.8c, a control C = HV connects the output Xo to+
VDD if the input is Xi=
LVorconnects Xo to ground if Xi=
HV. Thus, in the transfer mode, the transistors of a tri-state driver serve as transmission gates, thereby permitting an input signal to be enhanced (or refreshed); hence the meaning of the term driver. Of course, in the disconnect mode the tri-state driver produces a very large impedance (Hi-Z) between its input and output, virtually disconnecting the input from the output.Note that the conjugate logic circuit symbols are provided for each tri-state driver shown in 3.8 and that these symbols are interchangeable-as they must be, since they are derived from the same physical device (the tri-state driver). The idea here parallels that of the inverter and its conjugate logic circuit symbols shown in Fig. 3.6. Symbol
X
appearingC(H)
CMOS tri-state drivers, conjugate circuit symbols, and ideal equivalent circuits. (a) Noninverting tri-state driver with active high control, C. (b) Noninverting tri-state driver with active low control. (c) Inverting tri-state driver with active high control. (d) Inverting tri-state driver with active low control.
86
3.6 AND AND OR OPERATORS AND THEIR MIXED-LOGIC CIRCUIT SYMBOLOGY 87
on the output of an inverting tri-state driver in the transfer mode indicates an inverted voltage signal. Thus, if X is at LV, then
X
is at HV and vice versa.Buffers, or line drivers as they are sometimes called, may be composed of a series of inverters or gates used as inverters, or they may be simply a tri-state driver operated in the transfer mode. Remember, it is the function of a line driver to boost and sharpen signals that might otherwise degrade below switching levels or be distorted. The mixed logic circuit symbols for buffers are given in Fig. 3.20a.
3.6 AND AND OR OPERATORS AND THEIR MIXED-LOGIC