The article treats the topic of the energy consumption for a typical controller involved in smart sensor applications. Since in practice a common situation involves using off-the-shelf processors, in our case a Motorola HC908 family microcontroller, we concentrate on techniques leading to an optimisation at software level for low power requirements. The aim we envisage is to lower the energy consumption by means of due instruction selection and reordering, cycle and branch optimisation, and memory use such that without changing the original task a program performs, the energy consumed while executing it should decrease.