Black-Box Testing
Black-box tests are generated without knowledge of the code being tested. When used alone, black-box tests have a low probability of finding all the bugs in a program. But when used in conjunction with clear-box tests they help provide a well-rounded test set, since black-box tests are likely to uncover errors that are unlikely to be found by tests extracted from the code structure.

Blackbox tests can really work. For instance, when asked to test an instrument whose front panel was run by a microcontroller, one acquaintance of the author used his hand to depress all the buttons simultaneously. The front panel immediately locked up. This situation could occur in practice if the instrument were placed face-down on a table, but discovery of this bug would be very unlikely via clear-box tests.

One important technique is to take tests directly from the specification for the code under design. The specification should state which outputs are expected for certain inputs. Tests should be created that provide specified outputs and evaluate whether the results also satisfy the inputs.

We can't test every possible input combination, but some rules of thumb help us select reasonable sets of inputs. When an input can range across a set of values, it is a very good idea to test at the ends of the range.

For example, if an input must be between 1 and 10, 0, 1, 10, and 11 are all important values to test. We should be sure to consider tests both within and outside the range, such as, testing values within the range and outside the range. We may want to consider tests well outside the valid range as well as boundary-condition tests.

Random tests form one category of black-box test. Random values are generated with a given distribution. The expected values are computed independently of the system, and then the test inputs are applied. A large number of tests must be applied for the results to be statistically significant, but the tests are easy to generate.

Another scenario is to test certain types of data values. For example, integer-valued inputs can be generated at interesting values such as 0, 1, and values near the maximum end of the data range. Illegal values can be tested as well.

Regression tests form an extremely important category of tests. When tests are created during earlier stages in the system design or for previous versions of the system, those tests should be saved to apply to the later versions of the system. Clearly, unless the system specification changed, the new system should be able to pass old tests.

In some cases old bugs can creep back into systems, such as when an old version of a software module is inadvertently installed. In other cases regression tests simply exercise the code in different ways than would be done for the current version of the code and therefore possibly exercise different bugs.

Some embedded systems, particularly digital signal processing systems, lend themselves to numerical analysis. Signal processing algorithms are frequently implemented with limited-range arithmetic to save hardware costs. Aggressive data sets can be generated to stress the numerical accuracy of the system. These tests can often be generated from the original formulas without reference to the source