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Home > FPGA Technical Tutorials > FPGAs: World Class Designs > FPGA vs. ASIC Designs > REGISTER AND LATCH CONSIDERATIONS

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REGISTER AND LATCH CONSIDERATIONS

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Latches   

ASIC engineers often make use of latches in their designs. As a general rule-ofthumb, if you are designing an FPGA, and you are tempted to use a latch, don’t !  

Flip-flfl ops with both “ Set ” and “ Reset ” Inputs   

Many ASIC libraries offer a wide range of flip-flops, including a selection that  offer both set and reset inputs (both synchronous and asynchronous versions  are usually available).   By comparison, FPGA flip-flops can usually be configured with either a  set input or a reset input. In this case, implementing both set and reset inputs  requires the use of a LUT, so FPGA design engineers often try to work around  this and come up with an alternative implementation.

Global Resets and Initial Conditions   

Every register in an FPGA is programmed with a default initial condition (that  is, to contain a logic 0 or a logic 1).   Furthermore, the FPGA typically has a global reset signal that will return all  of the registers (but not the embedded RAMs) to their initial conditions. ASIC  designers typically don’t implement anything equivalent to this capability.


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