I am trying to get XST to infer an 8:1 or even a 4:1 mux, instead of using several 2:1 muxs'. Is there a suggested coding style to get xst to infer the larger muxes or how would i hardcode them to make larger muxes? Thanks Matt
how to get XST to infer 8:1 mux or just hard code it?
Started by ●April 7, 2004
Reply by ●April 7, 20042004-04-07
Usually your best chance of getting it is with a CASE statement. No guarantees though as synthesisers are notoriously unpredictable. You can also try structuring your VHDL to suggest a element layout. Instantiating macros in your HDL will give you a more exact structure. -- John Adair Enterpoint Ltd. http://www.enterpoint.co.uk This message is the personal opinion of the sender and not that necessarily that of Enterpoint Ltd.. Readers should make their own evaluation of the facts. No responsibility for error or inaccuracy is accepted. "Matthew E Rosenthal" <mer2@andrew.cmu.edu> wrote in message news:Pine.GSO.4.58-035.0404070358320.7554@unix3.andrew.cmu.edu...> I am trying to get XST to infer an 8:1 or even a 4:1 mux, instead of using > several 2:1 muxs'. > Is there a suggested coding style to get xst to infer the larger muxes or > how would i hardcode them to make larger muxes? > > Thanks > > Matt
Reply by ●April 7, 20042004-04-07
"John Adair" <newsreply@loseinspace.co.uk> wrote in message news:<XVOcc.2$bb6.0@newsr2.u-net.net>...> Usually your best chance of getting it is with a CASE statement. No > guarantees though as synthesisers are notoriously unpredictable. > > You can also try structuring your VHDL to suggest a element layout. > > Instantiating macros in your HDL will give you a more exact structure. > > -- > John Adair > Enterpoint Ltd. > http://www.enterpoint.co.uk > > This message is the personal opinion of the sender and not that necessarily > that of Enterpoint Ltd.. Readers should make their own evaluation of the > facts. No responsibility for error or inaccuracy is accepted. > > > "Matthew E Rosenthal" <mer2@andrew.cmu.edu> wrote in message > news:Pine.GSO.4.58-035.0404070358320.7554@unix3.andrew.cmu.edu... > > I am trying to get XST to infer an 8:1 or even a 4:1 mux, instead of using > > several 2:1 muxs'. > > Is there a suggested coding style to get xst to infer the larger muxes or > > how would i hardcode them to make larger muxes? > > > > Thanks > > > > Mattlook in the XST templates. In Verilog x <= (sel==0)? a0 : (sel==1)? a1 : (sel==2)? a2 : (sel==3)? a3 : an and so on works for me. I've gone to 8 no problem. But occasional use of hardcoding a MUXxyz can help to build the top stage if you need to combine say a 2->1 with an earlier 4->1 and so on. There are quite a few app nots in the xapp dir. regards johnjakson_usa_com
Reply by ●April 7, 20042004-04-07
I'm not certain about XST but my favorite trick with Synplify is to define
my selection as an 8-bit wire and select one of the bits. Simple! XST ?
(Verilog)
wire [2:0] Sel;
wire [7:0] MuxItems = {In1, InA, InB, Out1, Mid7, Result, whatA, whatB};
wire MuxOut = MuxItems[Sel];
"Matthew E Rosenthal" <mer2@andrew.cmu.edu> wrote in message
news:Pine.GSO.4.58-035.0404070358320.7554@unix3.andrew.cmu.edu...
> I am trying to get XST to infer an 8:1 or even a 4:1 mux, instead of using
> several 2:1 muxs'.
> Is there a suggested coding style to get xst to infer the larger muxes or
> how would i hardcode them to make larger muxes?
>
> Thanks
>
> Matt
Reply by ●April 7, 20042004-04-07
"Matthew E Rosenthal" <mer2@andrew.cmu.edu> wrote in message news:Pine.GSO.4.58-035.0404070358320.7554@unix3.andrew.cmu.edu...> I am trying to get XST to infer an 8:1 or even a 4:1 mux, instead of using > several 2:1 muxs'. > Is there a suggested coding style to get xst to infer the larger muxes or > how would i hardcode them to make larger muxes? > > Thanks > > MattI don't understand how you would make a 4:1 mux in a Xilinx without making it out of smaller muxes. The LUTs only have 4 inputs, so you can't make a 4:1 mux out of a single LUT. You need two LUTs plus another F5 mux. -Kevin
Reply by ●April 7, 20042004-04-07
Well, there are a few ways I can think of: 1) Use TBUFs to wire-OR LUT outputs together 2) Use the OR cascade in virtexII in the same way, preferable because it is faster and more plentiful 3) If you can accept a 16 clock set-up time, you can use the SRL16 as a programmable LUT. The programmer uses 2.5 slices, and loads up the SRL16 (or a word wide bank of them) with the appropriate pattern to connect the selected input to the output ( patterns are for a mux with enable are X"0000",X"AAAA",X"CCCC",X"F0F0",X"FF00"). This is useful for minimum propagation time 4:1 muxes for applications where the selection is relatively static, or is otherwise allowed time to complete. Yes, I have used it. 4) if your sources to the mux have available terms or come from flip-flops, you can substitute a 4 input OR for each MUX bit by having your select logic gate off all but one of the inputs at any given time. If the inputs are from flip-flops, you can use the sync reset on the flip-flops for the gate function without having to add logic in front of the flip-flop other than the decoder on the reset. Kevin Neilson wrote:> "Matthew E Rosenthal" <mer2@andrew.cmu.edu> wrote in message > news:Pine.GSO.4.58-035.0404070358320.7554@unix3.andrew.cmu.edu... > > I am trying to get XST to infer an 8:1 or even a 4:1 mux, instead of using > > several 2:1 muxs'. > > Is there a suggested coding style to get xst to infer the larger muxes or > > how would i hardcode them to make larger muxes? > > > > Thanks > > > > Matt > > I don't understand how you would make a 4:1 mux in a Xilinx without making > it out of smaller muxes. The LUTs only have 4 inputs, so you can't make a > 4:1 mux out of a single LUT. You need two LUTs plus another F5 mux. > -Kevin-- --Ray Andraka, P.E. President, the Andraka Consulting Group, Inc. 401/884-7930 Fax 401/884-7950 email ray@andraka.com http://www.andraka.com "They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759
Reply by ●April 8, 20042004-04-08
john jakson wrote:> > "John Adair" <newsreply@loseinspace.co.uk> wrote in message news:<XVOcc.2$bb6.0@newsr2.u-net.net>... > > Usually your best chance of getting it is with a CASE statement. No > > guarantees though as synthesisers are notoriously unpredictable. > > > > You can also try structuring your VHDL to suggest a element layout. > > > > Instantiating macros in your HDL will give you a more exact structure. > > > > -- > > John Adair > > Enterpoint Ltd. > > http://www.enterpoint.co.uk > > > > This message is the personal opinion of the sender and not that necessarily > > that of Enterpoint Ltd.. Readers should make their own evaluation of the > > facts. No responsibility for error or inaccuracy is accepted. > > > > > > "Matthew E Rosenthal" <mer2@andrew.cmu.edu> wrote in message > > news:Pine.GSO.4.58-035.0404070358320.7554@unix3.andrew.cmu.edu... > > > I am trying to get XST to infer an 8:1 or even a 4:1 mux, instead of using > > > several 2:1 muxs'. > > > Is there a suggested coding style to get xst to infer the larger muxes or > > > how would i hardcode them to make larger muxes? > > > > > > Thanks > > > > > > Matt > > look in the XST templates. > > In Verilog > > x <= > (sel==0)? a0 : > (sel==1)? a1 : > (sel==2)? a2 : > (sel==3)? a3 : > an > > and so on works for me. I've gone to 8 no problem. But occasional use > of hardcoding a MUXxyz can help to build the top stage if you need to > combine say a 2->1 with an earlier 4->1 and so on. There are quite a > few app nots in the xapp dir.I have not tried coding this in XST, but I am pretty sure it does not result in an 8:1 mux. The syntax you show is a priority selector. That means that it indicates an order of precedence. Even though it makes no difference since the selectors in each case are mutually exclusive, it is unlikely that this will be optimized to a proper mux. The recommended coding style would be a case statement, which by definition has mutually exclusive selections of a single control variable. A better coding method for bus muxes would be to pre-decode the selectors so that each of the 8 mux inputs has a separate enable. Then an 8 input mux can be done in just two levels of LUTs. The first level can encode the AND gates and one OR gate for combining two inputs. The second level is an four input OR gate for the final output. -- Rick "rickman" Collins rick.collins@XYarius.com Ignore the reply address. To email me use the above address with the XY removed. Arius - A Signal Processing Solutions Company Specializing in DSP and FPGA design URL http://www.arius.com 4 King Ave 301-682-7772 Voice Frederick, MD 21701-3110 301-682-7666 FAX
Reply by ●April 8, 20042004-04-08
When trying to create a mux with a case statement XST requires the output be a reg. Why is this and would making it a reg(latch) make the mux slower? Matt On Thu, 8 Apr 2004, rickman wrote:> john jakson wrote: > > > > "John Adair" <newsreply@loseinspace.co.uk> wrote in message news:<XVOcc.2$bb6.0@newsr2.u-net.net>... > > > Usually your best chance of getting it is with a CASE statement. No > > > guarantees though as synthesisers are notoriously unpredictable. > > > > > > You can also try structuring your VHDL to suggest a element layout. > > > > > > Instantiating macros in your HDL will give you a more exact structure. > > > > > > -- > > > John Adair > > > Enterpoint Ltd. > > > http://www.enterpoint.co.uk > > > > > > This message is the personal opinion of the sender and not that necessarily > > > that of Enterpoint Ltd.. Readers should make their own evaluation of the > > > facts. No responsibility for error or inaccuracy is accepted. > > > > > > > > > "Matthew E Rosenthal" <mer2@andrew.cmu.edu> wrote in message > > > news:Pine.GSO.4.58-035.0404070358320.7554@unix3.andrew.cmu.edu... > > > > I am trying to get XST to infer an 8:1 or even a 4:1 mux, instead of using > > > > several 2:1 muxs'. > > > > Is there a suggested coding style to get xst to infer the larger muxes or > > > > how would i hardcode them to make larger muxes? > > > > > > > > Thanks > > > > > > > > Matt > > > > look in the XST templates. > > > > In Verilog > > > > x <= > > (sel==0)? a0 : > > (sel==1)? a1 : > > (sel==2)? a2 : > > (sel==3)? a3 : > > an > > > > and so on works for me. I've gone to 8 no problem. But occasional use > > of hardcoding a MUXxyz can help to build the top stage if you need to > > combine say a 2->1 with an earlier 4->1 and so on. There are quite a > > few app nots in the xapp dir. > > > I have not tried coding this in XST, but I am pretty sure it does not > result in an 8:1 mux. The syntax you show is a priority selector. That > means that it indicates an order of precedence. Even though it makes no > difference since the selectors in each case are mutually exclusive, it > is unlikely that this will be optimized to a proper mux. > > The recommended coding style would be a case statement, which by > definition has mutually exclusive selections of a single control > variable. > > A better coding method for bus muxes would be to pre-decode the > selectors so that each of the 8 mux inputs has a separate enable. Then > an 8 input mux can be done in just two levels of LUTs. The first level > can encode the AND gates and one OR gate for combining two inputs. The > second level is an four input OR gate for the final output. > > -- > > Rick "rickman" Collins > > rick.collins@XYarius.com > Ignore the reply address. To email me use the above address with the XY > removed. > > Arius - A Signal Processing Solutions Company > Specializing in DSP and FPGA design URL http://www.arius.com > 4 King Ave 301-682-7772 Voice > Frederick, MD 21701-3110 301-682-7666 FAX >
Reply by ●April 8, 20042004-04-08
Why does it require a register? Yes it has to be in a process, but the process can be one sensitive to the mux inputs rather than to clock. You can also use the switch statement instead, which is a concurrent equivalent to the case. Matthew E Rosenthal wrote:> When trying to create a mux with a case statement XST requires the output > be a reg. Why is this and would making it a reg(latch) make the mux > slower? > > Matt > > On Thu, 8 Apr 2004, rickman wrote: > > > john jakson wrote: > > > > > > "John Adair" <newsreply@loseinspace.co.uk> wrote in message news:<XVOcc.2$bb6.0@newsr2.u-net.net>... > > > > Usually your best chance of getting it is with a CASE statement. No > > > > guarantees though as synthesisers are notoriously unpredictable. > > > > > > > > You can also try structuring your VHDL to suggest a element layout. > > > > > > > > Instantiating macros in your HDL will give you a more exact structure. > > > > > > > > -- > > > > John Adair > > > > Enterpoint Ltd. > > > > http://www.enterpoint.co.uk > > > > > > > > This message is the personal opinion of the sender and not that necessarily > > > > that of Enterpoint Ltd.. Readers should make their own evaluation of the > > > > facts. No responsibility for error or inaccuracy is accepted. > > > > > > > > > > > > "Matthew E Rosenthal" <mer2@andrew.cmu.edu> wrote in message > > > > news:Pine.GSO.4.58-035.0404070358320.7554@unix3.andrew.cmu.edu... > > > > > I am trying to get XST to infer an 8:1 or even a 4:1 mux, instead of using > > > > > several 2:1 muxs'. > > > > > Is there a suggested coding style to get xst to infer the larger muxes or > > > > > how would i hardcode them to make larger muxes? > > > > > > > > > > Thanks > > > > > > > > > > Matt > > > > > > look in the XST templates. > > > > > > In Verilog > > > > > > x <= > > > (sel==0)? a0 : > > > (sel==1)? a1 : > > > (sel==2)? a2 : > > > (sel==3)? a3 : > > > an > > > > > > and so on works for me. I've gone to 8 no problem. But occasional use > > > of hardcoding a MUXxyz can help to build the top stage if you need to > > > combine say a 2->1 with an earlier 4->1 and so on. There are quite a > > > few app nots in the xapp dir. > > > > > > I have not tried coding this in XST, but I am pretty sure it does not > > result in an 8:1 mux. The syntax you show is a priority selector. That > > means that it indicates an order of precedence. Even though it makes no > > difference since the selectors in each case are mutually exclusive, it > > is unlikely that this will be optimized to a proper mux. > > > > The recommended coding style would be a case statement, which by > > definition has mutually exclusive selections of a single control > > variable. > > > > A better coding method for bus muxes would be to pre-decode the > > selectors so that each of the 8 mux inputs has a separate enable. Then > > an 8 input mux can be done in just two levels of LUTs. The first level > > can encode the AND gates and one OR gate for combining two inputs. The > > second level is an four input OR gate for the final output. > > > > -- > > > > Rick "rickman" Collins > > > > rick.collins@XYarius.com > > Ignore the reply address. To email me use the above address with the XY > > removed. > > > > Arius - A Signal Processing Solutions Company > > Specializing in DSP and FPGA design URL http://www.arius.com > > 4 King Ave 301-682-7772 Voice > > Frederick, MD 21701-3110 301-682-7666 FAX > >-- --Ray Andraka, P.E. President, the Andraka Consulting Group, Inc. 401/884-7930 Fax 401/884-7950 email ray@andraka.com http://www.andraka.com "They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759
Reply by ●April 8, 20042004-04-08
