OFDT, 4, 8, 16

Single and Multiple D Flip-Flops with Active-Low 3-State Output Enable Buffers

OFDT, OFDT4, OFDT8, and OFDT16 are single or multiple D flip-flops whose outputs are enabled by a tristate buffers. The data outputs (Q) of the flip-flops are connected to the inputs of output buffers (OBUFT). The outputs of the OBUFTs (O) are connected to OPADs or IOPADs. These flip-flops and buffers are located in input/output blocks (IOB) for XC3000 and XC4000. The data on the data inputs (D) is loaded into the flip-flops during the Low-to-High clock (C) transition. When the active-Low enable inputs (T) are Low, the data on the flip-flop outputs (Q) appears on the O outputs. When T is High, outputs are high impedance (Off).

The flip-flops are asynchronously cleared with Low outputs, 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
T	D	C	O
1	X	X	Z
0	D		d
d = state of referenced input one setup time prior to active clock transition

Figure 8.29 OFDT Implementation XC4000, Spartans

Figure 8.30 OFDT Implementation XC5200, Virtex

Figure 8.31 OFDT Implementation XC9000

Figure 8.32 OFDT8 Implementation XC3000, XC4000, XC5200, XC9000, Spartans, Virtex