LDPE

Transparent Data Latch with Asynchronous Preset and Gate Enable

LDPE is a transparent data latch with asynchronous preset and gate enable. When the asynchronous preset (PRE) is High, it overrides the other input and presets the data (Q) output High. Q reflects the data (D) input while the gate (G) input and gate enable (GE) are High. 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 preset, output High, when power is applied. FPGAs simulate power-on when global set/reset (GSR) is active. GSR (XC4000X, SpartanXL, Virtex) default to active-High but can be inverted by adding an inverter in front of the GR/GSR input of the STARTUP or the STARTUP_VIRTEX symbol.

Inputs				Outputs
PRE	GE	G	D	Q
1	X	X	X	1
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 7.15 LDPE Implementation XC4000X, SpartanXL