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XST takes a very long time to complete synthesis, why?

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Junior Member level 2
Oct 21, 2005
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xst problem

i have just bought a spartan3 starter kit.iam currently developing the 8051 microcontroller core in vhdl so far i have completed all instructions except a few such as div ab . the problem is xst takes a very long time to complete synthesis,
the code is about 1300 lines,it takes about 20 min to complete one byte instructions
when i include 2 and 3 byte instructions the synthesis goes on for 8 hours and still doesn't complete.i'm fairly new to vhdl coding so i don't know whether the code i've written is inefficient or something else
iam also including the vhdl code
it has 3 files one(rev1_1.vhd) has the actual vhdl code for 8051
the other two are for ram and rom.
if anyone could help it would be great

xst problem

well you have written a very high level code, thats way its taking time. ireg is written in very high-level description, that might also causing problem. split the design into datapath and control-unit. draw a datapath block diagram of your core -- showing alu, register file, temp-regs, data_ram and code-rom, muxes (where needed) and their interconnection -- in other words for rtl think in terms these components. then draw controller fsm -- which will generate control signals.

other issues are avoid type "bit", "bit_vector" use "std_logic" & "std_logic_vector" instead, as it not suitable for both simulation & synthesis. avoid shared variables, use signals.

see these cores (links below) as well, first one is very good quality.

see books like "vhdl coding style" by ben cohen (see coding for synthesis), "Digital Logic and Microprocessor Design with VHDL" by Enoch Hwang,
"vhdl programming by example" by Douglas Perry, "rapid prototyping of digital systems" by hamblen and furman. and comp-arch book like "computer orginization hardware/software interface" by hennesy and patterson. (some of these books are on edaboard).


(hope this helps!)

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