UMBC CMSC 313, Computer Organization & Assembly Language, Spring 2002, Section 0101
Finite State Machines
Tuesday 04/23, 2001
Assigned Reading in Murdocca & Heuring: A.12-13
Assigned Reading in Neveln:
- Recap flip-flops.
- On flip-flops and latches: the terminology for flip-flops and
latches have not been standardized. What one textbook calls a latch
another may call a flip-flop. Our textbook (M&H) calls everything a
flip-flop. Here's what some other textbooks use. Both latches and
flip-flops refer to sequential circuit constructs that "remember"
or "record" a certain input signal at some point in time.
- A latch is often used for circuits where the output
changes right after the input changes, without reference to any
clocking event. What our text calls an SR flip-flop is often
called an S-R latch in other textbooks.
- A level-sensitive latch is a latch that operates
only when the clock is high or only when the clock is low.
What our text calls a clocked SR flip-flop is called a
level-sensitive SR latch in some other texts.
- The term flip-flop is often reserved for
circuits that record the input only during certain clocking
events. The expectation is that the output does not also change
during this clocking event. The master-slave flip-flops described
in our text-book certainly fits this category.
- An edge-triggered flip-flop is a flip-flop that only records
the input during a low-to-high (positive edge triggered) or
a high-to-low (negative edge triggered) transition of the clock.
The input signal must be stable for a duration (the setup
time) prior to the clock transition and must remain stable for
a duration called the hold time. There is some disagreement
on whether a master-slave flip-flop is an edge-triggered flip-flop.
My interpretation is that it is not. Our textbook uses the
symbol for an edge-triggered flip-flop for the master-slave D
flip-flop. Another textbook does the same thing, but does mention
that some people do not consider master-slave flip-flops to be
edge-triggered. Still another textbook puts master-slave flip-flops
in a category all by itself. (One problem here is that one might
reasonably argue that a master-slave D flip-flop is negative
edge triggered, but a master-slave JK flip-flop really isn't
- Examined a race condition that might occur while putting
together a master-slave D flip-flop.
- Examined an (honest to goodness) negative edge triggered D flip-flop.
- Constructed a Mod-4 counter.
- Discussed DigSim Exercise 2.
Circuits in DigSim:
Following these links will launch the DigSim Java applet and
load the circuit described.
14 Nov 2003 10:17:29 EST
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