Upper management actually asked me to share my TDD experience as well & so I just published an article internally to our Embedded Software newsletter describing how TDD helped my project. Here’s the summary from that article (I think the dates really say it all):
Here is a good question, and my reply, from a recent attendee of my Test-Driven Development for Embedded C training.
As I work more with TDD, one of the concepts I am still struggling to grasp is how to test “leaf” components that touch real hardware. For example, I am trying to write a UART driver. How do I test that using TDD? It seems like to develop/write the tests, I will need to write a fake UART driver that doesn’t touch any hardware. Let’s say I do that. Now I have a really nice TDD test suite for UART drivers. However, I still need to write a real UART driver…and I can’t even run the TDD tests I created for it on the hardware. What value am I getting from taking the TDD approach here?
(this article is not just for embedded developers)
A few months back I was reading Doug Schmidt’s blog, on The Growing Importance of Sustaining Software for the DoD, and he made the claim
“although software does not wear out, firmware and hardware become obsolete, thereby requiring software modifications”.
It was a bit of a clarifying moment for me. Doug clarified two terms that I suppose were obvious, or maybe not. Software is this thing that can have a long useful life, where firmware is going to be regularly be made obsolete as hardware evolves.
I’d like to add to Doug’s statement:
although software does not wear out, it can be destroyed from within by unmanaged dependencies on firmware and hardware.
Think of all the code denied the potential long life of software due to being infected with dependencies on hardware.
One of the attendees of my training objected to TDD stating “TDD does not resolve the real-world (temperature, pressure, timing, noisy signals, etc.) issues that my project is encountering.”
You are right! I’ll add TDD does not resolve anything. TDD is not a magic incantation that solves any problem the embedded developer may encounter. From discussions at your company, I think you realize this. But it does not change the fact that you have to spend a lot of time chasing these kinds of problems. So let’s see how TDD can support this activity.
I’m going to bend the example a little, and look at how to write tests that interact with an interrupt service routine and again with a semaphore. I’ll also get some help with the tedious parts of building fakes by using Mike Long’s Fake Function Framework. (BTW: I am using a beta version, that has changes the released version does not yet have.)
This figure shows the concurrent runtime relationship between the
MessageProcessor and its
In the last article, the
OSSemPend() test-double was coded to handle a specific
OSSemPend() application and test need. The semaphore was being used to signal when there is a message to process. It was the first need for a
OSSemPend() test double and was quickly developed. As more RTOS dependent code is brought under test, a more general solution will be needed.
In this article, we’ll look at a test double that can be customized for each application.
Some silicon vendors extend the C language so the programmers can easily interact with the silicon. Using these extensions tie production code to the silicon vendors compiler and consequently the code can only run on the target system. This is not a problem during production, but is a problem for off-target unit testing.
The good news is that we may be able to get around this problem without having to change production code, one of our goals when adding tests to legacy code.
My last article featured a hand crafted a spy to monitor
asm directives. Now let’s use CppUMock (the mock support companion CppUTest) to create a mock version of
Sometimes embedded developers have to use inline assembler instructions to get better control of the processor, or to improve performance. How should we deal with those when we’re doing TDD and testing off the target?
What’s the problem? The embedded
asm statements cause compilation errors if the assembler instructions are not part of the off-target test platform instruction set. Also some of the instructions might not be legal in the test environment. This article shows how to insert a test double for the
asm directives with gcc and CppUTest.
At the start of a new development effort, there is considerable uncertainty. There are unknowns in hardware, software, product goals and requirements. How can we get started with all this uncertainty? Isn’t better to wait? If you wait, there really is no end to the waiting, so its better to get started sooner even though there will be some things you decide early that get changed later.