Page 137 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

118 Chapter Eleven K5 Latch -set NAND

IT'
I
+ 4,
-set
-set

-reset
-reset
-reset

NAND

- reset - set I %,,+I -% n+ 1
I
0 0 I* 1" q = (-set & -4)
0 1 0 1
1 0 1 0 -q = (-reset & q)
1 1

(I" = Unstable %ate) Figure 1 1-1 2. NAND implementation
of an RS latch

In a NAND implementation, both the -reset and -set inputs are active
low, as is indicated by the bobbles associated with these inputs on the symbol
and by the tilde characters in their names. As a reminder, if any input to a
NAND is 0, the output is 1, and it's only if both inputs to a NAND are 1 that
the output will be 0. Working out how this version of the latch works is left as
an exercise to the reader.6

D-Type Latches

A more sophisticated function called a D-type ("data-type") latch can be
constructed by attaching two ANDs and a NOT to the front of an RS latch
(Figure 11-13).

6 This is where we see if you've been paying attention <grin>.