Page 136 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Using Primitive Logic Functions to Build More Complex Functions w 1 17

Both Reset and Set Active Reset and Set Go Inactive

Figure 11-1 1. RS latch: the reset and set inputs go inactive simultaneously

goes to 1, and this value is fed back to the input of the first gate. Each gate
now has its fed-back input at I, and both gates therefore attempt to drive their
outputs to 0. As we see, the circuit has entered a metastable condition in which
the outputs oscillate between 0 and 1 values.
If both halves of the function were exactly the same, these metastable
oscillations would continue indefinitely. But there will always be some
differences (no matter how small) between the gates and their delays, and the
function will eventually collapse into either its reset condition or its set condition.
As there is no way to predict the final values on the s, and -4. outputs, they are
indicated as being in X, or don't know, states (PI and 130). These X states
will persist until a valid input sequence occurs on either the reset or set inputs.
An alternative implementation for an RS latch can be realized using two
NAND gates connected in a back-to-back configuration (Figure 11-12).