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From: Symon on 17 Apr 2008 14:06 Joerg wrote: > > > Yep, that's why I usually do not do my own layouts. Occassionally I > route a small portion of a circuit and send that to my layouter. No > DRC or anything, just to show him how I'd like it done. > Hi Joerg, That's a excellent middle way. Cheers, Syms.
From: Andy on 17 Apr 2008 14:21 On Apr 17, 1:04 pm, "Symon" <symon_bre...(a)hotmail.com> wrote: > Dave wrote: > > Does anybody out there have a good methodology for determining your > > optimal FPGA pinouts, for making PCB layouts nice, pretty, and clean? > > The brute force method is fairly maddening. I'd be curious to hear if > > anybody has any 'tricks of the trade' here. > > > Also, just out of curiosity, how many of you do your own PCB layout, > > versus farming it out? It would certainly save us a lot of money to > > buy the tools and do it ourselves, but it seems like laying out a > > board out well requires quite a bit of experience, especially a 6-8 > > layer board with high pin count FPGA's. > > > We're just setting up a hardware shop here, and although I've been > > doing FPGA and board schematics design for a while, it's always been > > at a larger company with resources to farm the layout out, and we > > never did anything high-speed to really worry about the board layout > > too much. Thanks in advance for your opinions. > > > Dave > > Hi Dave, > I layout my own PCBs. Unlike Mike T., I don't let the FPGA tools pick the > pinout. That said, it is important to consider carefully consider nets which > might have tight timing, e.g. clocks. I reason that there is a lot more > flexibility in the FPGA routing than on my PCB, and it's cheaper, so I can > save most time and money by being flexible in the pinout. I set the banks > the nets are to go on, and firm up the detailed pinout by swapping pins on > the FPGAs banks during the PCB layout process. You need some experience in > what your HDL code is gonna look like to be able to do this, but there you > go. > If you are adept at FPGA work, you'll find learning a PCB layout tool is a > piece of cake. I also use laser drilled microvias from layer 1 to 2, which > make the layout of big BGAs easier and saves layers. SI is easier also. The > price is usually less this way; the layers outweigh the via expense. You > don't need buried vias, IME. > Some of my FPGA buddies and I have had bad experiences with contract PCB > people. Sometimes they are knowledgeable and talented, but sometimes they > are dogmatic idiots, and sometimes they are useless. If you go the contract > route, it's important to closely monitor what they get up to so you find out > early doors which type they are. > Like you and Mike say, it depends a lot on your experience. If you've worked > closely with your layout guys in the past, that'll be a big help to you. > For sure, there's more than one way to skin a cat, but I enjoy PCB layout. > YMMV, good luck with it. > Cheers, Syms. > p.s. One benefit to laying out the PCB yourself is that it can help you spot > stupid mistakes in the circuit as you go. It forces you to look very closely > at the layout. Depending on your PCB layout (and schematic capture) tools' capabilities for defining constraints on pin swappability, you can develop symbols that constrain IO pin swapping to meet the needs of the design and/or FPGA. For example, we have symbols for FPGA's that limit IO pin swapping to the same bank and other banks powered from the same voltage rail. We lock down critical pins (global clock inputs, etc.) and we have to "seed" the banks with their voltage assignments, but after that, we are often able to let the PWB tool auto-swap the FPGA pins, and then clean that up in layout. Then we feed that pin out back to the FPGA design tools, and make sure we can place and route the design in the FPGA while meeting timing. This is often with a preliminary version of the FPGA code, but with relevant IO structures in place. Andy
From: Symon on 17 Apr 2008 14:30 Andy wrote: > assignments, but after that, we are often able to let the PWB tool > auto-swap the FPGA pins, and then clean that up in layout. Then we > feed that pin out back to the FPGA design tools, and make sure we can > place and route the design in the FPGA while meeting timing. This is > often with a preliminary version of the FPGA code, but with relevant > IO structures in place. > > Andy Hi Andy, Do you use PADS I/O designer? Also, have you ever found that the PWB tool swapped pins that prevented your FPGA code meeting timing? I never bother testing, because it always does. I'd be interested in any counter examples you have. Thanks for your post, Syms.
From: John Adair on 17 Apr 2008 14:36 Dave We are slightly unusual in that we started as FPGA design house and now probably better known for our boards even though we do an awful lot of internal FPGA design still. A lot of board layout is just common sense. Having a plan of how it all fits together - not just placement but routing runs between chips usually pays great dividends. Having someone who understands both the FPGA and the pcb layout is usually a great advantage as it allows tradeoffs to be made easily and generally ends up with with a better board. Swaping I/Os as you layout will give a much better results. That all said we are still learning on our pcb design skills even after producing development boards for nearly 5 years and I can still say generally that every new board we do is technically better than the previous one we did. Your first board will probably take a long time especially if it as in any way complex. Our first development board (Broaddown2 for the interested) that we released took about 800hrs of man effort. We would do that same board now in probably less than 1/3 of that time now. So in summary you have the difficult decision whether to invest time in learning the trade, making mistakes along the way, and possibly getting better boards versus the direct cost of using someone experienced and reducing the risks of a good enough to ship first layout. Very few people achieve boards that are good enough to ship as practical production boards as first revisions and if you do that you are doing well. Wire mods etc in production cost lots. I'm know of some designs done by customers themselves that have gone to 7 versions due to mistakes in layout. That's not cheap and really hits timescales. I'm proud to say my team have delivered over 50% of our development boards to production, to ship at 1st issue, but that is definately unusual in boards of that level of complexity. Board can be an enjoyable task but it's not for the impatient. John Adair Enterpoint Ltd. On Apr 17, 5:43 pm, Dave <dhsch...(a)gmail.com> wrote: > Does anybody out there have a good methodology for determining your > optimal FPGA pinouts, for making PCB layouts nice, pretty, and clean? > The brute force method is fairly maddening. I'd be curious to hear if > anybody has any 'tricks of the trade' here. > > Also, just out of curiosity, how many of you do your own PCB layout, > versus farming it out? It would certainly save us a lot of money to > buy the tools and do it ourselves, but it seems like laying out a > board out well requires quite a bit of experience, especially a 6-8 > layer board with high pin count FPGA's. > > We're just setting up a hardware shop here, and although I've been > doing FPGA and board schematics design for a while, it's always been > at a larger company with resources to farm the layout out, and we > never did anything high-speed to really worry about the board layout > too much. Thanks in advance for your opinions. > > Dave
From: Steve on 17 Apr 2008 17:13
"Joerg" <notthisjoergsch(a)removethispacbell.net> wrote in message news:U5MNj.6956$GE1.6193(a)nlpi061.nbdc.sbc.com... > qrk wrote: >> On Thu, 17 Apr 2008 09:43:09 -0700 (PDT), Dave <dhschetz(a)gmail.com> >> wrote: >> >>> Does anybody out there have a good methodology for determining your >>> optimal FPGA pinouts, for making PCB layouts nice, pretty, and clean? >>> The brute force method is fairly maddening. I'd be curious to hear if >>> anybody has any 'tricks of the trade' here. >>> >>> Also, just out of curiosity, how many of you do your own PCB layout, >>> versus farming it out? It would certainly save us a lot of money to >>> buy the tools and do it ourselves, but it seems like laying out a >>> board out well requires quite a bit of experience, especially a 6-8 >>> layer board with high pin count FPGA's. >>> >>> We're just setting up a hardware shop here, and although I've been >>> doing FPGA and board schematics design for a while, it's always been >>> at a larger company with resources to farm the layout out, and we >>> never did anything high-speed to really worry about the board layout >>> too much. Thanks in advance for your opinions. >>> >>> Dave >> >> Sure wish there was a slick way of doing FPGA pinouts. I usually use >> graph paper and figure out the FPGA pinout to other parts to minimize >> routing snarls. >> >> I do pcb layouts on my own and other folks designs. Our boards have >> high-speed routing, switching power supplies, and high-gain analog >> stuff; sometimes all on the same board. Unless the service bureau has >> someone who understands how to lay out such circuitry and place >> sensitive analog stuff near digital junk, it is more trouble to farm >> out than do it yourself if you want the board to work on the first >> cut. >> > > Or find a good layouter and develop a long-term business relationship. My > layouter knows just from looking at a schematic which areas are critical. > He's a lot older than I am and that is probably one of the reasons why his > stuff works without much assistance from me. Nothing can replace a few > decades of experience. > > >> Doing your own layout will take a lot of learning to master the PCB >> layout program and what your board vendor can handle. It will take 5 >> to 10 complicated boards to become mildly proficient at layout. I >> don't know about saving cost. Your time may be better spent doing >> other activities rather than learning about layout and doing the >> layouts. ... > > > Yep, that's why I usually do not do my own layouts. Occassionally I route > a small portion of a circuit and send that to my layouter. No DRC or > anything, just to show him how I'd like it done. > > >> ... The upside to doing your own layout - you control the whole >> design from start to finish. If you have a challenging layout, you'll >> have a much higher probability of having a working board on the first >> try which has hidden savings (getting to market earlier <- less >> troubleshooting + less respins). >> >> --- >> Mark > > > -- > Regards, Joerg > > http://www.analogconsultants.com/ > > "gmail" domain blocked because of excessive spam. > Use another domain or send PM. I agree with Joerg. Good high speed or mixed signal PCB layout is a career choice, and we electrical engineers already chose our career. A good layout requires someone who understands not just the software package, but the details of how the manufacturing operation is going to proceed, what the limits of the processes are, what the assembly operations require of the board, and is anal about things like footprint libraries and solder mask clearances and a thousand other details that I'm only partially aware of. The more complex your design, the more critical these things become. I have two good local outfits for farming out boards. For complex stuff, they know I'll come to their place and sit next to the designer for a good bit of the initial placement. While we are doing placement, we are also discussing critical nets, routing paths, layer usage, etc. That gives us direct face to face communication and avoids spending lots of time trying to write/draw everything in gory detail (which gets ignored or misunderstood a lot of the time). That investment pays big dividends in schedule and board performance. Don't be fooled by the relatively low cost of the software. That's not where the big costs are. I once laid off my entire PCB layout department and sent all the work outside, because although my employees all knew how to use the software, none of them could tell me what their completion date would be, or how many hours it would take, and they certainly weren't interested in meeting schedules. The outside sources would commit to a cost and a delivery date. And we already knew they could meet our performance objectives. Fixed price contracts are great motivators. Missing an engineering test window, or slipping a production schedule because of a layout delay can be enormously expensive. Of course, if I had let my engineers do their own layouts, the motivation would have been present, but the technical proficiency would not. How proficient can anyone become if they only do layout a few times a year? Also, on many projects engineers use the layout period for other important things like documentation, test procedures, writing test code, etc. Doing your own layout serializes these tasks and will stretch your schedule. So my advice is to keep doing what you have been doing. Its far more likely that its the cheapest approach, even though you occasionally have to write a big check. Steve |