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U. Glaeser

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FIGURE 10.59 Precharged TSPC transparent-high<br />

blocking latch with embedded NOR2 logic.<br />

FIGURE 10.60 A positive, edge-triggered flip-flop built from transmission gate latches.<br />

One of the advantages of the precharged TSPC latch is that, similar to Domino, relatively complex<br />

logic can be incorporated in the precharge stage. An example of a latch with an embedded NOR2 is given<br />

in Fig. 10.59.<br />

Although this latch cannot be used generically because of its special input requirement, it is the base<br />

of a TSPC flip-flop (discussed next) and of pulsed flip-flops described later on in this chapter section.<br />

Design of Flip-Flops<br />

This sub-section explains the fundamentals of flip-flop design. It covers three types of flip-flops based<br />

on the transmission gate, tristate, and TSPC latches presented earlier. The sense-amplifier based flip-flop,<br />

with no latch equivalence, is also discussed. Design trade-offs are briefly mentioned.<br />

Master-Slave Flip-Flop<br />

The master-slave flip-flop, shown in Fig. 10.60, is perhaps the most commonly used flip-flop type [6].<br />

It is made from a transparent-high and a transparent-low transmission gate latch. Its mode of operation<br />

is quite simple: the master section writes into the first latch when CK is low, and the value is passed onto<br />

the slave section and propagated to the output when CK is high. As pointed out earlier, a flip-flop made<br />

this way has to satisfy the internal min-timing requirement. Specifically, the delay from CK to X has to<br />

be greater than the hold time of the second latch. Notice that the second latch turns completely opaque<br />

only after CKB goes low. The inverter delay between CK and CKB creates a short period of time where<br />

both latches are transparent. Therefore, designers must pay careful attention to the timing of signals X<br />

and CKB to make sure the design is race free. Setting the min-timing requirement aside, the master-slave<br />

flip-flop is simple and robust; however, for applications requiring very high performance, its long D-to-<br />

Q latency might be unacceptable.<br />

A flip-flop made from tristate latches (see Fig. 10.55), that is, free of internal races, yet uses complementary<br />

clocks, is shown in Fig. 10.61 [16]. The circuit, also known as C 2 MOS flip-flop, does not require<br />

the internal inverter at node X because: (1) node X drives transistor gates only, so there is no fight with<br />

a feedback inverter, and (2) there is no internal race: a pull up(down) path is followed by a pull down(up)<br />

path, and both paths see the same clock. The D-to-Q latency of the C 2 MOS flip-flop is about equal or<br />

better than the master-slave flip-flop of Fig. 10.60; however, because of the stacked PMOS devices, this<br />

circuit dissipates more clock power and is less area efficient. For the same reason, also the input load is<br />

higher.<br />

© 2002 by CRC Press LLC<br />

D<br />

CKB<br />

CK<br />

CK<br />

X<br />

Q<br />

D1 D2 X<br />

Q<br />

QB

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