Re: Getting started with bare-board development

["Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>]

On 11/12/2016 09:14 AM, Adam Jensen wrote:
>
> How is it done in embedded software engineering? (Links and/or
> references are very welcome)!

Typically embedded S/W has to interface to various H/W devices (sensors and 
actuators). Frequently such S/W is designed around the capabilities and features 
of the intended H/W. This is not a good idea. When the intended H/W changes (as 
it does frequently on the projects I've been involved in) the entire design has 
to be revised.

What I have done when designing such S/W is to 1st design the core S/W without 
regard to the capabilities and features of the intended H/W. I create the 
simplest and clearest design, and this identifies the kind of information the 
S/W needs to obtain and the kind of external actions it needs to take.

Next, for each piece of external information the S/W needs to obtain, I write a 
pkg spec for a S/W-leaning interface. This keeps the S/W simple and clear by 
providing just the kind of I/F it needs.

Then, for each intended H/W device, I write a pkg spec for a H/W-leaning 
interface. This reflects the capabilities and features of the device.

Then I write bodies for each of the S/W I/F pkgs that use the H/W I/F pkgs.

Now comes the fun part. I write an environment pkg that simulates reality, and 
write simulation bodies for the H/W I/F pkgs that read or modify that simulated 
reality. The body can take do things to make its behavior realistic; for 
example, if a sensor is noisy, the body would add noise to the real value.

This lets you play with your S/W and see if it behaves reasonably.

When it's time to run the S/W on the real system, you eliminate the environment 
pkg and replace the H/W I/F bodies with ones that actually I/F with the H/W. 
Note that the only differences between the simulated and actual systems are 
those bodies.

This approach has a number of benefits:

* Changing a device only affects a S/W I/F body and a H/W I/F pkg.

* Often the simplest and clearest design for the core S/W wants to access 
information or take action in a way the intended H/W doesn't support. The S/W 
I/F pkg provides a single place to convert between the 2 views, keeping the core 
S/W uncoupled. For example, in the ubiquitous cruise-control problem, the best 
approach for the core S/W might be for it to decide when it obtains the car's 
speed, but a common design for the speed sensor is something that generates an 
interrupt every time something rotates a certain amount.

* While there is usually a 1:1 correspondence between S/W and H/W I/F pkgs, 
there need not be. I've seen sensors that returned multiple, unrelated values. 
The design had multiple S/W I/F pkgs interacting with a single H/W I/F pkg.

* I've worked on projects where the whole point was to create a simulation to 
see if the approach is viable, with no idea what the H/W devices would be like 
in a real system. By using this approach, when it was decided to go ahead with a 
real system, only the H/W I/F pkgs and the S/W I/F bodies had to be rewritten.

When I present such a design, coders usually start whining about "efficiency". 
In my decades of experience, such a design has never been responsible for a 
system not meeting its timing requirements.

-- 
Jeff Carter
"Many times we're given rhymes that are quite unsingable."
Monty Python and the Holy Grail
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