Vitaliy wrote:
> "Loose coupling" means having as few connection points between program
> elements, as possible. "Connection points" are functions and
> variables that a module exposes to the outside world, or variables
> that are shared between two or more functions or modules.
>
> "Tight cohesion" is how you get the simple and clean interface for
> both functions and modules. A function should only do what its name
> implies. Not more, not less. A module should only contain functions
> that directly relate to the module's "job description".
>
> If properly followed, these rules help ensure that you can reuse the
> code with a minimal amount of pain.

Those things generally help, but there's more to it than that.  On a small
embedded system there is no operating system, so code may have to make
assumptions about how certain parts of the hardware are used, and about
overall project firmware structure.  Even well written code can be
incompatible and difficult to integrate into another project.

For example, one project may use a master event loop.  Another project may
use simple threads.  A subsystem may be designed quite differently depending
on which model it assumes is in use.  Making it general may not be easy,
either requiring a mess of build-time switches or excessive runtime bloat.
Your project may need to reserve interrupts for a particular low latency
task, but a off the shelf subsystem uses interrupts in a integral way.
Changing its overall structure may well be harder than creating a subsytem
designed from the beginning to not use interrupts.  Then there is allocation
of limited hardware resources that need to be planned at the project level.
A off the shelf subsystem may have been designed assuming it can have a
hardware resource to its exclusive use.


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