Which spot should I test for effective embedded software testing?

Today, the embedded industry is changing fast - systems have become larger, more complex, and more integrated. Embedded system consists of heterogeneous layers such as hardware, HAL, device driver, OS kernel, and application. These heterogeneous layers are usually customized for special purpose hardware. Therefore, various hardware and software components of embedded system are mostly integrated together under unstable status. That is, there are more possibilities of faults in all layers unlike package software. In this paper, we propose the embedded software interface as two essential parts: interface function that represents the statement of communication between heterogeneous layers, and interface variable that represents software and/or hardware variable which are defined in different layer from integrated software and used to expected output for decision of fault. Also, we statically investigate various views of embedded software interface and demonstrate that proposed interface should be new criterion for effective embedded software testing.