Working software, iteratively

27 April 2015
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Reading time: 5 minutes

How does scrum framework work in an embedded systems programming environment?

My first post advocated adopting an agile approach to embedded software development. One agile principle is breaking up projects that are over a month or so in duration into a series of smaller deliveries. These deliveries might be suitable for a customer to assess or even actively use, or might be intended only for internal validation. They allow progress to be measured by demonstrating working code and allow closer customer collaboration to verify that the product will meet their need. Breaking functionality across iterations focuses the team on the immediate delivery and allows changes to be accommodated in future iterations with relative ease.

The scrum project management framework is a well-proven approach for iterative development. In scrum, the work to be undertaken is divided into smaller pieces, preferably as user stories capturing a user-testable feature, but often as a task listing derived from a work breakdown structure. These work items are prioritized in a backlog. Items to be completed are selected at the start of each iteration (aka, sprint), highest priority first. Sprint lengths are preferably fixed and short; four weeks or less is common.

Scrum recommends that each sprint deliver working software. This is considered impractical by many teams, putting some off of scrum completely. However working software does not have to mean delivery of a fully functional release, or even the minimum viable product. And in an embedded environment working software in early iterations may well rely on mocks and stubs of yet to be delivered hardware.

IEC 62304, an ISO standard for medical device software lifecycle processes, notes three feasible development strategies:

Waterfall. Development proceeds once through a single development cycle.
Incremental. System requirements are defined in full up front, delivery of these requirements is then broken in to a sequence of iterations.
Evolutionary. System requirements are only partially defined up front; each iteration includes requirement refinement work.

Sample project

For this post I’ll provide an example of a project where we’ve followed an incremental strategy, and how Scrum was a good fit (Scrum also fits naturally within an evolutionary approach, but that’s for another post).

The customer in this case, a developer of medical diagnostic devices, has an existing product with a mix of data sources and targets connecting over a variety of hardware interfaces. The data is subject to a number of data processing algorithms and the results are displayed on a touch screen. A complete software re-architecture was required to enable the device to support a number of new data sources and to make it possible to use a single codebase on multiple new platforms and operating systems.

The requirements, hazard analysis, and safety cases were already in place for the existing system, so the scope for the rebuild was well understood. Stakeholders, naturally cautious of an entire redesign, needed early assurance that the new software would deliver the improvements and flexibility required.

The overall development was allocated to a number of monthly iterations, with common (highly depended upon) and hardware-agnostic code scheduled for early sprints. An initial sprint planning meeting is held for each iteration, where the features to be delivered are agreed and captured in a backlog. This listing, commonly captured in a tool such as Jira or a spreadsheet, is a living artefact visible to the team and stakeholders throughout.

Some features are labelled low priority for a particular iteration and only implemented if time allows. The allocation of features to be delivered in future sprints is only very roughly planned. Where features are yet to be started the customer can swap in alternates or change priorities as needed.

Short (15 minute) daily scrum meetings are held where each team member summarizes the work that he or she has done since the last meeting and intends to complete before the next. Any blocking issues are raised also. The customer provided a product owner who attends all scrum meetings; her role is crucial in agreeing feature details or advising on prioritizations, and she has excellent visibility of progress. Care needs to be taken to ensure that the scrum meeting does not become a daily design meeting; such discussions should be kept separate as they rarely require the whole team.

In the first month, we delivered architectural services implementing cross cutting concerns such as persistence, fault management, logging, and an operating-system abstraction layer. Delivery in this context means that the software features were designed with the design documented in UML and text, production code and unit tests were written and reviewed, unit-testing coverage was measured on all target operating systems, and Doxygen was used to provide API and test-code documentation. By the end of the iteration the customer had seen all these aspects demonstrated, and a continuous integration server (we use Jenkins with CxxTest) gives everyone visibility that the code compiles and all tests pass whenever the code base changes.


An iterative approach provides all with high visibility of a project’s progress, delivers greater value and higher risk features earlier, and allows for requirements to evolve as the development proceeds which results in a better overall solution.

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