Answers Database

XC3000A/XC4000: How to implement flip-flops with both asynchronous preset and clear/reset inputs


Record #2632

Product Family:  Hardware

Product Line:  4000

Problem Title:
XC3000A/XC4000:	How to implement flip-flops with both asynchronous
preset and clear/reset inputs



Problem Description:
Keywords:  flip-flop, register, preset, reset, clear

Urgency: standard

General Description:
The Xilinx XC3000/A/L and XC4000/E/EX/XL architectures do not
support flip-flops with both PRESET and RESET inputs.  How
do you implement this functionality?


Solution 1:

Since neither architecture supports flip-flops with both SET
and RESET pins, there is no efficient way to implement such
a function.  The best one can do is mimic such a configuration,
using two flip-flops (one with a direct PRESET input, and
the other with a direct RESET input) in conjunction with a MUX
which you would use to select the appropriate flop, depending
on which function was desired at any given time.




End of Record #2632