FDC

D Flip-Flop with Asynchronous Clear

FDC is a single D-type flip-flop with data (D) and asynchronous clear (CLR) inputs and data output (Q). The asynchronous CLR, when High, overrides all other inputs and sets the Q output Low. The data on the D input is loaded into the flip-flop when CLR is Low on the Low-to-High clock transition.

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. FPGAs simulate power-on when global reset (GR) or global set/reset (GSR) is active. GR for XC3000 is active-Low. GR for XC5200 and GSR (XC4000, Spartans, Virtex) default to active-High but can be inverted by adding an inverter in front of the GR/GSR input of the STARTUP or STARTUP_VIRTEX symbol.

Inputs			Outputs
CLR	D	C	Q
1	X	X	0
0	1		1
0	0		0

Figure 5.10 FDC Implementation XC3000, XC4000, XC5200, Spartans

Figure 5.11 FDC Implementation XC9000