What goes into motherboard design?

[CTho9305]

It can't be as simple as designing a board for a simple 20MHz PIC... what is involved? I was talking to Mark R (who I think is not in the industry, but seems very knowledgeable), and he said he thinks there is more to it than putting a piece of metal between point a and point b... but you also need other traces to improve signal quality, and I guess stuff like inductors and capacitors.

When building a board for a 20MHz PIC, it is simple... wire from point to point and be done. Obviously motherboards aren't this simple or anyone could make their own for cheap. And what makes two motherboards with the same chipset perform differently? Again, on the PIC board, the trace either conducts properly or doesn't. There is no "slower", ESPECIALLY since it is clocked. If after 1/2*10^-6 seconds the signal isn't clear, stuff stops working. However, any review shows that different motherboards with the same chipset perform at different (but close) speeds.

 

[ProviaFan]

I have no idea how to answer your questions, but they are very well thought out and might derive an interesting answer from some of the highly knowledgeable people around here, so I'm bumping your thread for you and subscribing to it at the same time so I can be updated about its contents.

 

[Mday]

the biggest concern is the quality of the components. you may have read about popping capacitors in some products.

most companies are given reference boards and schematics for chipsets today. they are also given problems to be overcome (which are not publicly acknowledged). thus, many are simply putting out the same reference board. And others do minor changes to make their board unique, ie, adding extras.

cost is the primary motivation in motherboard design. sure they want a good product, but some things are sacrificed. they jam everything into as little an area as possible. furthermore, if they decide to make several versions based on teh same chip, they will often use the same layout plus or minus some components for features. that's why you will often see missing slots or chips where there is space allocated.

one of the items to overcome is noise. there are many components on a motherboard that will interfere with each other. such items are generally placed far apart as to prevent interference. and filters and such are added to prevent any problems from arising. trace length is also of concern. the shorter the trace, the cheaper it is. but some traces have to be long. this is where chip placement is key.

back to the reference design, since reference designs are so common, you will notice that boards dont really perform that differently, manufacturer to manufacturer. any differences are neglible. this is also why i consider component quality to be the primary concern, since some boards dont deviate much from the reference design.

 

[CTho9305]

But as I said in my post, bad traces will just make my PIC stop working. It won't slow down by 1%. There is definitely more going on here.

 

[Peter]

Those tiny performance differences come from different choices and setup of clock synthesizers, and the zillions of performance related timing registers found in today's chipsets. Most of the time it's the clock chips though.

 

[bzbee]

Mday is right about cost being the primary motivation for device selection, but there are often other considerations. As already mentioned EMI is a big issue, and is true of any consumer electronics device requiring FCC certification. Often there are thermal considerations, eg I'm trying to fit this in a pizza box sized chassis, which can only withstand 30W of thermal dissipation, and if I don't want to have a fan, I have to be careful picking parts. Real estate is an important issue too in this case (no pun intended). Most designers also try to get as much integration as possibly for a couple of reasons, higher integration often means lower bill of materials cost, less board area/fewer layers means less PCB cost, and fewer interfaces for the designer to debug. A PIC project is very much different from a PIC-based product, since a product usually requires some form of UL/surge testing, emissions certification and quality control. That's where the various caps and ESD/filter circuitry comes in. It's all about making a cheaply manufactured design that is reliable (ie few board failures on the manufacturing floor), is usable in multiple designs through stuffing options, and is least likely to explode and cause lawsuits

 

[SuperTool]

Having done a little mobo design during a summer internship, I remember that a lot of free food, drinks, smoothies, freebies, and intern trips to the amusement park going into it.
I also remember playing around with service processors, ASICS, beds of nails, microsoldering, scopes, etc.
It's interesting if you like hands on stuff. I personally preffer digital VLSI design, because ratio of technical work to paperwork seems to be higher. Also, I like running SPICE way more than trying to hold my hand steady to read a signal off the motherboard.
As far is why one board is faster than other, higher quality boards allow more agressive timing settings. If you have less noise and faster traces, you can adjust timing on some circuits. It will also be more stable, because there is less chance of interference. The more layers you have the easier it is to route traces, and the more places you can put shielding, and the bigger power/ground planes you have, and you can make some critical traces wider (lower resistance). Also, you need capacitors to provide good solid ground and power signals to all chips which could draw high currents.

 

[Turbonium]

What exactly is PIC?...

 

[CTho9305]

Originally posted by: Protocell
What exactly is PIC?...

Programmable Interrupt Controller. Basically a very simple processor, in this case with some analog outputs and PWM (used to control motors).

 

[sao123]

I believe the process for a motherboard goes somehting like this...

1)Digital experts work to create the data flow patterns between components during the design phase. This is a difficult step because MoBo's Memory and new Generation CPU's are generally in design at the same time, and all new non-peripheral communication technologies are added at this phase. The three teams must work together in sync. They test its pulse, clock timing and flow and logic for all circuits.

2)After the digital design is finished, then the design is sent off to the EE's to turn the digital logic into real electrical circuits. These are the people who determine which components will work in a circuit to create the digital circuit. They also are responsible for determining resistance, voltage, current, capacitance, at every point in the board at any given clock cycle.

3)The board is built tested for both correctness of all digital logic, and electric specs.

4)After the main functionality is done, all the extras are added. Ports like parallel, serial, usb, ATA-HDD & Floppy controller, LED's etc.
New peripheral technologies can be added here.

5)More circuit testing is done.

6)Next the BIOS is coded and added.

7)More testing... most testing is done here.. usually a year.

then the board is ready for production.
8)Dies are cast and factories built...mass quantities are shipped.

 

[casper911]

you could say that motherboard functionality is chips dependance but performance is vary board by board.

reference design guide just advise you what u should do. not all the boards could follow exactly.

so how is the performance difference?
routing is the key for signal quality and a well power delivery. bad layout may make the board not stable, position of caps on board, the trace location all determine the signal quality. high speed signal should not go beside critical signal like clock, some power signal and so on. number of vias, width of a trace is also important. some even need a differential pair signal... bla bla bla...