Page 632 - Engineering Digital Design

602 CHAPTER 12 / MODULE AND BIT-SLICE DEVICES

(c)
FIGURE 12.39
Three examples of memory elements suitable for use in a ripple counter, (a) FET JK flip-flop in toggle
mode, (b) FET T flip-flop in toggle mode, (c) RET D flip-flop wired as a toggle module.

in Fig. 12.39c is the least costly (hardware-wise) of the three and is the one featured in the
ripple counter of Fig. 12.38a, but as an FET toggle module.

Take Care in Using the Ripple Counter There are two major problems that can arise in
using ripple counters. The problems are stated as follows together with some suggestions
for proper use:

• All ripple counters suffer from a progressive noise (glitch) generation problem
if any attempt is made to decode their outputs. An inspection of the timing
diagram in Fig. 12.38b shows the transition delays that result from the series
(ripple) triggering of the flip-flops. If decoding of the outputs is not necessary or
if decoding is used but glitch production can cause no problem, ripple counters
can be used advantageously — they require no external logic for their operation.
If these conditions cannot be met, no attempt should be made to use ripple
counters. Instead, use should be made of synchronous binary counters whose
output transitions are synchronous or very nearly so.
• Ripple counters are inherently slow compared to synchronous counters. This is
so because the output changes must propagate through the counter one flip-flop
at a time. For an n-bit ripple counter this propagation delay may be expressed as

T Ripple Counter = « X ?ff , (12.14)

where rg is the delay through a single flip-flop in the ripple counter. This counter
delay would be required for completion of a 2" binary count. In comparison,
the delay of a divide-by-2" synchronous binary counter required to complete the
count is

*Synch Counter = (tff + tNs) (12.15)

and is not progressive. In Eq. (12.15) TNS is the propagation delay through the
next-state-forming logic required in the design of the synchronous counter.

Therefore, if counter speed is not a consideration and if the outputs are not decoded in
any way, use of ripple counters can be recommended. In fact, if these conditions apply,