Ernie & Duracell
"I feel great" - long pause - "I feel great, I feel great".
6 weeks later: "I feel great, I feel great, I feel great" - pause - "I feel great".
I hear this sound coming out of my son's room. What is going on in my house? Is there such a thing as too much euphoria? No, sometimes my son does utter this phrase, but most of the time it is coming from an Ernie toy he inherited from his cousin several years ago.
This particular toy is over 14 years old and still "feels great". We had it for over 5 years. So, by now the batteries should have given up. Nonetheless, we still get these random, out of the blue utterances of the phrase: "I feel great". This is supposed to be triggered by some sort of child-toy interaction. However, it mutated into sound generation at random intervals. This phenomenon is very bizarre, as the pauses are very long, causing completely random operation.
The other day the toy went off again: "I feel great". My suspicion was that this might wake up my son in the middle of the night. As Ernie's electronics and wiring obviously have some issues, and as Ernie does not have an on/off switch, my first recourse was to remove the batteries.
To my surprise, the batteries are more than 10 years old and we have never replaced them since we received the toy. The circuit is not supposed to draw a lot of current. However, it is always on and ready to "speak". Overall, this is pretty amazing battery longevity - Hats off to Duracell!
Moving From Sesame Street to the Real World
In verification, your goal is to put the DUT into all known scenarios, and as many unknown scenarios as possible. Constrained random test generation is particularly helpful in achieving the latter. In e [IEEE 1647] constrained random generation is front and center. It is a core principal of the e language and the associated methodology. By default, everything, every aspect and every field, is random in e. This will ensure that you get to as many unknown and unanticipated scenarios to test your device as thoroughly as possible, and that you identify the associated bugs during simulation.
When everything is random by default you are not at risk of forgetting to randomize any aspects. Consequently, you are less dependent on the quality of your coverage model to detect flaws in the test generation ability of your environment.
JL Gray, VP of Verilab North America puts it like this : "Setting up a verification environment where you have to decide what not to randomize ends up being far more randomized in the end than one where you have to decide what to randomize."
Another effect of default randomization is that it enables early bug detection. More randomization shakes out more bugs. Since detecting bugs early is less costly than detecting them later, this has a positive impact on the overall verification cost.
Default randomization does not imply that you bring up your environment with the wildest transactions imaginable. You still control what you want to see initially. However, it does mean that you are typically moving to a high level of randomization more quickly.
This principal of a default randomized environment, is called Infinity Minus (∞-) and is illustrated by the following code:
// apb_trans_s.e (abridged file) - sequence item definition
struct apb_trans_s like any_sequence_item {
addr : uint;
data : uint;
direction : read_write_t;
delay : uint;
...
}
In this simplified and abridged example of an APB transaction definition, all fields are randomized by default. In other words, you get random addresses combined with random data, random direction (read or write), and random delays between transactions. Subsequently, you impose rules by adding constraints and reigning in the randomization to suit your particular testing needs.
E * D ≈ ∞- Ernie * Duracell ≈ Infinity Minus
The broken electronics in the Ernie toy, and the almost infinite longevity of the Duracell batteries, are behaving just like they are driven by an infinity minus stimulus generator. Ernie feels great at random times, but you will feel great all the time knowing your designs have been verified using the infinity minus verification approach.
Keep on verifying!
Axel Scherer
Incisive Product Expert Team
Twitter, @axelscherer