bibo1978
Joined: 01 May 2004
Posts: 229
Helped: 6
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 22 Jun 2004 17:25 vhdl lookup table |
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a look up table "LUT" is actually an Sram or a flash memory (or at least the first LUT was), most LUTs nowaday are 4X1 which means that they are capable of implmenting any 4 -input to 1-output function. "4-address RAM with one bit o/p".
Virtually any combinational logic can be implemented with cascaded and parallel LUTs.
for example if you have a 16 case statement with only one o/p that have a discrete value ('1' or '0') at each statement this can be implemented in 1 LUT "which is by the way a ROM".
Note that in FPGAs LUTs doesn't exist alone they usually exists with other configurable blocks which may contain FF, MUXs, other connection defining modules "not related to routing matrix" and arithematic acceleration logic.
this Configurable Logic Blocks are named "CLB", the CLB may contain several LUTs, FFs, MUXs and arithemtic acceleration logic.
In VHDL you can usualy instantiate or infer any element the FPGA offers including LUT, any compinational statements can be implemented with LUT "doesn't mean that they actually is implemented in LUT", to instantiate a LUt all what you do is describe the data in the RAM "uaully in hexadecimal" but this is the hardway on the other hand you can just let the synthesis tool configure the data of the LUT.
Some extra features may be added to a LUT for example it can be configured to work as a "ROM, RAM and even sometimes as a shift register"
thats all folks,
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