Lut in fpga form1/21/2024 ![]() ![]() If I could fetch two instructions at a time, that wouldĬost only 60 clocks, and yet provide me with two instruction words. So it was always fetching a new instruction after executing every instruction. Logic, the flash actually ran at half that speed, costing meĤ4 clocks every time I wished to fetch an instruction. ![]() Since I was generating a clock signal from The next 8 to allow the flash to look up your address, and then anotherĮight to read the data back out. ![]() The first six of those are for the address, I wanted to upgrade the fetch routine, so that it could handle fetching twoīecause of the way the QSPI flash is set up, the first word you fetch from One using and one without using these instructions. Instructions, then I wouldn’t need to compile two versions of the C-library: (sometimes) pack two instructions into one instruction word. To be able to use the compressed instruction LH (load halfword), SH (store halfword), LB (load byte), and SB (store byte). The obvious: I wanted to be able to support the extra 8-bit byte instructions: Took 52 clocks per instruction when running from the flash. Hence, when I started my upgrades, the ZipCPU Because the S6/LX4 has hardlyĪny RAM, almost all of the instructions had to fit within the flash. Upgrades surrounded the slow flash speed. Not only did I want the new instructions toįit, but I wanted to make some other upgrades as well. The additional logic necessary to fit the new instructions fit onto the With only 2400 six-input LUTs to work with in total, and only about 50 to The problem with this was that the ZipCPUĪlready worked and worked well on some very small FPGA’s, and the Problems I was going to have, I then decided to byte the bullet and add Figuring that this was only the tip of the iceberg regarding the In my case, the light dawned when I realized that I would need to rewrite theĮntire C-library to support 32-bit bytes, thatĨ-bit bytes. Or when computer code depends upon 4*sizeof(char)=sizeof(int). This has all kinds of consequences when you wish to operate on 8-bit values, Unit of individually addressable memory was the full bus size: 32-bits. To get the most logic for a given price, the FPGA design engineer needs to beĪble to code efficiently, and pack their code into the fewest LUTs possible. In general, the more LUTs you have, the more logic your chip can do,īut also the more your FPGA chip is going to cost. The point being that every FPGA implements your logic via a combination ofĬhips differ by the capability of their LUTs, as well as by the number of LUTs On the other hand, has only 4-input LUTs. LUT’s can handle either one six input lookup, or two five input lookups–as Which each of those “slices” contain four 6-input LUTs. “configurable logic blocks”, each of which contain two “slices”, of Using a lookup table, you can buildĪny logic you want–so long as you don’t exceed the number of elements in theĪs an example, the 7-series Xilinx FPGAs are composed of Several strategies for reducing my LUT count on an FPGA. Last Lookup-Table (LUT) out of your FPGA. ![]() I typically can’t afford a bigger FPGAĪt times like these, a thorough code scrubbing can often help you squeak that Building or even justĪssembling a new board with a bigger FPGA can be an expensive proposition. With the FPGA resources that you already have. Production, and you wish to add features to the board, you will be stuck If you already have your board designed, built, and tested–perhaps even in This answer is problematic, though, for two primary reasons: Manager how to deal with this problem, they will think you need a bigger FPGA. There have now been several projects I’ve worked on where I’ve butted upĪgainst the maximum resource utilization of an FPGA chip. ![]()
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