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D<br />

DCK<br />

FIGURE 10.73 Latches and flip-flops with scan circuitry.<br />

both transmission gates are buffered. The circuitry that controls the DCK and SCK is not shown for<br />

simplicity. In terms of speed, drain/source and gate loading is added to nodes X and Q. This increases<br />

delay slightly, although less substantially than adding a full multiplexor at the latch input. A similar<br />

approach to the one just described may be used with the TSPC latch, as suggested in Fig. 10.73(b). Since<br />

transistor P 1 is not guaranteed to be off when DCK is low, transistor P 2 is added to pull-up node X during<br />

scan mode. Control signal SEB is the complement of SE and is set low during scan operation.<br />

Figure 10.73(c) shows the scannable version of the master-slave flip-flop. This design uses three clocks:<br />

CK (free running clock), DCK (data clock), and SCK (scan clock). DCK and SCK, when enabled, are<br />

complementary to CK. Under data or scan mode, CK is always toggling. In scan mode, DCK is driven<br />

low and SCK is enabled. In data mode, SCK is driven low and DCK is enabled. While this approach<br />

minimizes the number of scan related devices, its drawback is the usage of three clocks. One clock may<br />

be eliminated at the expense of increased scan complexity. If CK and DCK are made fully complementary,<br />

and CK is set low during scan mode (DCK is set high), the same approach used in Fig. 10.73(a) may be<br />

used.<br />

Figure 10.73(d) shows a possible implementation of an scannable sense amplifier flip-flop (see Fig. 10.63).<br />

In scan mode, DCK is set low forcing nodes X and Y to pull high. The output latch formed by the cross<br />

coupled NANDs is driven by the scan flip-flop. To ensure the latch can flip during scan, the NAND gate<br />

driving QB must either be weak or be disabled by SCK. Also for robustness, node QB should be kept<br />

internal to the circuit.<br />

Figure 10.73(e) shows the scannable version of the transmission gate pulsed latch of Fig. 10.66(a). The<br />

circuit requires the generation of two pulses, one for data (DPCK) and one for scan (SPCK). It should<br />

be noticed that having pulsed latches in the scan chain might be deemed too risky because of min-timing<br />

problems. To make the design more robust, a full scan flip-flop like in Fig. 10.73(a) shall be used instead.<br />

The disabling of the data (or scan) path in a pulsed latch during scan (or data) mode does not require<br />

the main clock to be disabled. Instead, the delayed clock phase used in the pulse generation can be<br />

disabled. This concept is used in the implementation of scan for the pulsed latch of Fig. 10.67, and it is<br />

shown in Fig. 10.73(f). As previously explained, this latch uses embedded pulse generation. Signals SE<br />

© 2002 by CRC Press LLC<br />

D<br />

SI<br />

D<br />

SI<br />

DCK<br />

SCK<br />

SEB<br />

P2 X<br />

DCK<br />

SCK<br />

X<br />

(a)<br />

P 1<br />

(b)<br />

(c)<br />

Q<br />

Q<br />

CK<br />

SO<br />

QB<br />

SO<br />

QB<br />

SI SO<br />

SCK SCK<br />

D<br />

SI<br />

DPCK<br />

SPCK<br />

D<br />

DCK<br />

CK<br />

(e)<br />

X<br />

Q<br />

Y<br />

QB<br />

DB weak<br />

SI SO<br />

(d)<br />

D<br />

SI<br />

DCKB<br />

SE<br />

N N<br />

1 1<br />

X<br />

SEB<br />

D SI<br />

(g)<br />

SCKB<br />

Q<br />

SCK SCK<br />

SE<br />

SEB<br />

(f)<br />

QB<br />

CK<br />

QB<br />

SO

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