Page 152 - Engineering Digital Design

PROBLEMS 123

3.7 Use the definitions of positive and negative logic and Fig. 3.6 as a guide to construct
the physical truth table and its two mixed logic truth tables for each of the tri-state
drivers in Fig. 3.8 as listed below. Remember to include both inputs Xi and the control
C. Use the letter D to represent the disconnect mode.
(a) Noninverting tri-state driver with C(H).
(b) Noninverting tri-state driver with C(L).
(c) Inverting tri-state driver with C(H).
(d) Inverting tri-state driver with C(L).
3.8 By adding an inverter to each, reconfigure the tri-state drivers in Figs. 3.8c and 3.8d
so that they become noninverting tri-state enable switches with driver capability. Give
the circuit symbols and ideal equivalent circuits for these two reconfigured tri-state
switches.

3.9 Reconfigure the NAND gate in Fig. 3. lOa by flipping it end-for-end such that the two
series NMOS are on the +VDD (HV) end and the two parallel PMOS on the ground
(LV) end.
(a) Construct the physical and mixed logic truth tables for this reconfigured CMOS
circuit. Is this a valid gate form and, if so, what logic function does it perform?
(Hint: Compare with Fig. 3.16.)
(b) What, if any, are the disadvantages of this new configuration? Explain.
3.10 Repeat Problem 3.9 for the NOR gate in Fig. 3.12a, but with the two parallel NMOS
on the HV end and the series PMOS on the LV end. (Hint: Compare with
Fig. 3.17.)

3.11 Explain why the AND and OR gates of Figs. 3.16 and 3.17 cannot be used for
logic level conversion as is done for the NAND and NOR gates of Figs. 3.14 and
3.15.

3.12 Write the logic expressions for the action indicated by the situations given below. Use
mnemonics or abbreviations where appropriate.
(a) Bob will go fishing in a boat only if the boat does not leak and if it is not windy.
Otherwise, he will fish from the bank, but only if the fish are biting.
(b) A laboratory class consists of five students (A, B, C, D, and E) each from a dif-
ferent discipline. An experiment has been assigned that must be carried out with
any one of the following combinations of students:

A and C but not D
A or B but not both (see Section 3.9)
D but only if E is present

(c) A robot is activated only if a majority of its three switches (X, Y, and Z) are turned
ON and is deactivated if a majority of its three switches are turned OFF.

3.13 Archie (A), Betty (B), Cathy (C), and David (D) may attend a school dance, but will
dance only with the opposite sex and then only under the following conditions: Archie
will dance with either Betty or Cathy. However, Cathy will dance with Archie only