LDCE

Transparent Data Latch with Asynchronous Clear and Gate Enable

LDCE is a transparent data latch with asynchronous clear and gate enable. When the asynchronous clear input (CLR) is High, it overrides the other inputs and resets the data (Q) output Low. Q reflects the data (D) input while the gate (G) input and gate enable (GE) are High and CLR is Low. If GE is Low, data on D cannot be latched. The data on the D input during the High-to-Low gate transition is stored in the latch. The data on the Q output remains unchanged as long as G or GE remains low.

The latch is asynchronously cleared with Low output when power is applied. FPGAs simulate power-on when global reset (GR) or global set/reset (GSR) is active. GR (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, STARTUP_SPARTAN2, or STARTUP_VIRTEX symbol.

Inputs				Outputs
CLR	GE	G	D	Q
1	X	X	X	0
0	0	X	X	No Chg
0	1	1	0	0
0	1	1	1	1
0	1	0	X	No Chg
0	1		D	d
d = state of input one setup time prior to High-to-Low gate transition

Figure 7.11 LDCE Implementation XC4000X, SpartanXL