One that works.... Forever....So what is a good design?
Yes, that is the big one, and is the overriding criteria from which all other criteria ripple down.One that leaves satisfied customers.
I designed a display panel... I was lucky that I designed it ( completely by accident) so its uses seem endless..
I had used a 40 pin pic and routed every pin so it could have future use... I seem to add and add more and more features.. There doesn't appear to be any digital / analogue cross talk... There doesn't seem to be any excess noise issues... I powered it with a known SMPS design that is very stable... The screen is connected such that large knocks / vibration give little issue and I can stick the complete display in 1 meter of water!!!
I didn't expect any of this... Doubt if I could do it again... I'm only on the third PCB revision!!
Luck!!!
When I started... It was proven on stripboard... Because of this tracks are kept short as possible as stripboard rearranging is my speciality.. the loadcell amp and the analogue inputs are on one side of the pic and all the digital signals are on the other... When I moved to home brew PCB's I kept the same format, so the very first professional board worked straight of the bat... I suppose the most important design aspect is the separation of analogue / digital components... I learned this from the complete bozo who laid out the last board for my last employers... And he was vastly more qualified than me... Oh and I also grasped the horrible design flaws of ground bounce very early on.... I spent an age wondering why a crystal wouldn't start oscillating.. A 4meg worked.. An 8meg worked, but the 12meg just wouldn't!!! Placing of crystals and stabilisation is the first lesson...
I hear the question " Why is my DS1307 time inaccurate".. "Because you strapped the crystal on crossing data lines..."
So on hind sight... Maybe I have a little idea about design..
Unpacking a little bit, and getting down to brass tacks...
A good design:
Is economical to produce
Has a high yield in production
Gives the customer a high ratio of value to price
Works properly (i.e., meets its published specs) regardless of environmental conditions
Once in production, require little or no Engineering intervention
Is insensitive to component variations and aging
Doesn't rely on components meeting their "typical" data sheet specs
Is insensitive to interference (EMI)
Is well protected against ESD
Operates components well inside their maximum ratings, plenty of safety margin
Is reliable (i.e., low failure rate)
Is as simple as it can possibly be, but no simpler
Is easy to use, with a user interface that's as intuitive as possible
Has good user documentation
Has good design documentation
Has minimal susceptibility to component obsolescence
I'm sure others can cite other factors that make for a good design, but those are the things that come to mind right now.
I think sometimes it means "more complex than it needs to be." Other times, it seems to mean "has far more capabilities than what was requested."You often hear the word, 'overkill' used on ETO in reference to a design- anyone know what that means?
This fundamentally important design criteria has some serious ramifications.Doesn't rely on components meeting their "typical" data sheet specs
This fundamentally important design criteria has some serious ramifications.
Take a circuit board with a hundred TTL chips on it. The typical current consumption per chip may be 50mA per chip, but the worst-case current consumption may be 150mA.
Assuming typical current consumption you get a 5V line current capability of 50mA * 100 = 5A, but taking worst-case you get a current capability requirement of 100 * 150mA =15A. Which current capability target do you aim for?
If a data sheet doesn't give any value at all for a particular parameter, I either don't use that part or I make sure my design will work over any plausible, non-absurd range of values for that parameter. In the case where only a "typical" value is cited, I'll usually guesstimate a range of values to work from in checking my design, provided the parameter is not critical. If it is critical, I simply won't use the part.Also, what to do if a component data sheet only gives a typical value or, worse still, no value at all for a particular parameter.
An excellent answer- sometimes you have to be pragmatic.Good point. This is one of those times where "don't rely on 'typical' specs" has to be gamed a bit, and tempered with experience and judgement since it is very unlikely that all of the chips on a given board will draw the worst-case current.
If a data sheet doesn't give any value at all for a particular parameter, I either don't use that part or I make sure my design will work over any plausible, non-absurd range of values for that parameter. In the case where only a "typical" value is cited, I'll usually guesstimate a range of values to work from in checking my design, provided the parameter is not critical. If it is critical, I simply won't use the part.
Good grief...Talking about relying on data sheet parameters, I once came across a circuit where the designer was using the inverting and non-inverting input bias current of an opamp to generate an offset voltage.
Nothing has been overlooked- we are having a discussion about design not a specific design.One point that should be made, and that often seems to be overlooked here,
Oh yes there is.is that there is no single best design.
I can't see anywhere where anybody has said otherwise.There is more than one way to accomplish a task, and just because somebody is using a different approach than you, that doesn't necessarily make it the wrong approach.
Quite so. That is a tenant for good design. That is all that is needed to be said. Nobody is criticizing your designs.Much like Ian said, in my own designs, I try to take a wider outlook than the immediate project. Adding a few extra pads to a circuit board and maybe a few extra components may expand the board's applications and allow it to solve future problems.
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