FDCPE

D Flip-Flop with Clock Enable and Asynchronous Preset and Clear

FDCPE is a single D-type flip-flop with data (D), clock enable (CE), asynchronous preset (PRE), and asynchronous clear (CLR) inputs and data output (Q). The asynchronous PRE, when High, sets the Q output High; CLR, when High, resets the output Low. Data on the D input is loaded into the flip-flop when PRE and CLR are Low and CE is High on the Low-to-High clock (C) transition. When CE is Low, the clock transitions are ignored.

The flip-flop is asynchronously cleared, output Low, when power is applied. For CPLDs, the power-on condition can be simulated by applying a High-level pulse on the PRLD global net. Virtex and Spartan2 simulate power-on when global set/reset (GSR) is active. GSR defaults to active-High but can be inverted by adding an inverter in front of the GSR input of the STARTUP_SPARTAN2 or STARTUP_VIRTEX symbol.

Inputs					Outputs
CLR	PRE	CE	D	C	Q
1	X	X	X	X	0
0	1	X	X	X	1
0	0	0	X	X	No Chg
0	0	1	0		0
0	0	1	1		1

Figure 5.14 FDCPE Implementation XC9000