Testing State Machine Flow Without External Hardware - c

Question

I'm currently in the process of writing a state machine in C for a microcontroller (a TI MSP430). Now, I don't have any problems with writing the code and implementing my design, but I am wondering how to prove the state machine logic without having to use the actual hardware (which, of course, isn't yet available).
Using debugging features, I can simulate interrupts (although I haven't yet tried to do this, I'm just assuming it will be okay - it's documented after all) and I have defined and reserved a specific area of memory for holding TEST data, which using debugging macros, I can access at runtime outside of the application in a Python script. In other words, I have some test foundations in place. However, the focus of my question is this:
"How best do I force a certain state machine flow for decisions that require hardware input, e.g., for when an input pin is high or low". For example, "if some pin is high, follow this path, otherwise follow this path".
Again, using debugging macros, I can write to registers outside of the application (for example, to light an LED), but I can't (understandably) write to the read-only registers used for input, and so forcing a state machine flow in the way described above is proving taxing.
I had thought of using #ifdefs, where if I wanted to test flow I could use an output pin and check this value instead of the input pin that would ultimately be used. However, this will no doubt pepper my codebase with test-only code, which feels like the wrong approach to take. Does anyone have any advice on a good way of achieving this level of testing? I'm aware that I could probably just use a simulator, but I want to use real hardware wherever possible (albeit an evaluation board at this stage).

Answer 1

Sounds like you need abstraction.
Instead of, in the "application" code (the state machine) hard-coding input reading using e.g. GPIO register reads, encapsulate those reads into functions that do the check and return the value. Inside the function, you can put #ifdef:ed code that reads from your TEST memory area instead, and thus simulates a response from the GPIO pin that isn't there.
This should really be possible even if you're aiming for high performance, it's not a lot of overhead and if you work at it, you should be able to inline the functions.

Answer 2

Even though you don't have all the hardware yet, you can simulate pretty much everything.
A possible way of doing it in C...
Interrupt handlers = threads waiting on events.
Input devices = threads firing the above events. They can be "connected" to the PC keyboard, so you initiate "interrupts" manually. Or they can have their own state machines to do whatever necessary in an automated manner (you can script those too, they don't have to be hardwired to a fixed behavior!).
Output devices = likewise threads. They can be "connected" to the PC display, so you can see the "LED" states. You can log outputs to files as well.
I/O pins/ports can be just dedicated global variables. If you need to wake up I/O device threads upon reading/writing from/to them, you can do so too. Either wrap accesses to them into appropriate synchronization-and-communication code or even map the underlying memory in such a way that any access to these port variables would trigger a signal/page fault whose handler would do all the necessary synchronization and communication for you.
And the main part is in, well, main(). :)
This will create an environment very close to the real. You can even get race conditions!
If you want to be even more hardcode about it and if you have time, you can simulate the entire MSP430 as well. The instruction set is very compact and simple. Some simulators exist today, so you have some reference code to leverage.
If you want to test your code well, you will need to make it flexible enough for the purpose. This may include adding #ifdefs, macros, explicit parameters in functions instead of accessing global variables, pointers to data and functions, which you can override while testing, all kinds of test hooks.
You should also think of splitting the code into hardware-specific parts, very hardware-specific parts and plain business logic parts, which you can compile into separate libraries. If you do so, you'll be able to substitute the real hardware libs with test libs simulating the hardware.
Anyhow, you should abstract away the hardware devices and use test state machines to test production code and its state machines.

Answer 3

Build a test bench. First off I recommend when for example you read the input registers or whatever, use some sort of function call (vs some volatile this that the other address thing). Basically everything has at least one layer of abstraction. Now your main application can easily be lifted and placed anywhere with test functions for each of the abstractions. You can completely test that code without any of the real hardware. Also once on the real hardware you can use the abstraction (wrapper function, whatever you want to call it) as a way to change or fake the input.
switch(state)
{
case X:
r=read_gpio_port();
if(r&0x10) next_state = Y;
break;
}
In a test bench (or even on hardware):
unsigned int test_count;
unsigned read_gpio_port ( void )
{
test_count++;
return(test_count);
}
Eventually implement read_gpio_port in asm or C to access the gpio port, and link that in with the main application instead of the test code.
yes, you suffer a function call unless you inline, but in return your debugging and testing abilities are significantly greater.