On 10/14/2010 9:46 AM, scott wrote:

> On our boards nearly all of the discrete components are part of the
> power supply or to round off the data signals to improve EMI/EMC
> performance.

[A large chunk of this response is mostly directed at invisible]

Right. Filtering the signals. Caps are used a lot of time in digital 
circuits to prevent switching noise from corrupting signals via ground. 
When some digital circuits (TTL in particular) switch they create a lot 
of noise on the ground plane that must be dealt with. Too much 
fluctuation between power and ground, and the circuit begins to behave 
erratically. Also, its a good idea to limit current. Resistors are used 
to stabilize signals, pull up or pull down an input or output, bleed off 
excess voltage so FETs don't retain a charge, etc... You need RC 
networks to run a clock crystal, etc... Just connecting together a pile 
of digital components won't work. You need supporting components to make 
sure everything is stable in the system, especially when dealing with 
large-scale digital systems like an iPhone. Keep in mind, that device is 
also going to require a oscillators and filters for the RF parts, as well.

It gets even more interesting when you start dealing with designing 
circuit boards, where you actively need to avoid traces from running 
parallel to each other for too much distance, lest they build a 
parasitic capacitance between them. In systems with very tight voltage 
and current tolerances, the length of traces and thickness of traces 
becomes very important as well. Traces can also act as inductors.

Some of the discrete components may be in there to also counteract some 
of these effects caused by the traces themselves on the board. You'll 
notice caps and resistors will often be very close to the pins of an IC 
to minimize any effect the circuit trace may have.

-- 
~Mike

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