The practical applications of wireless sensor networks require the sensor devices to be high in computation ability, low in power consumption, small in size, as well as competitive in cost. In addition, there are other particular attributes which require special consideration in sensor device design. These include, for example, the capabilities in protecting the embedded program from foreign intrusion and supporting efficient air-programming, which are very common requirements in many large-scale applications. In this paper, we describe the system architecture and design methodology of an ASCI-based sensor network device to meet those attributes for a class of applications. Compared with existing works, the unique features of the sensor device include: (1) a module-based extensible processor and co-processor cooperation architecture is designed with application-specific components to accelerate the signal collection, processing, and networking via hardware-software co-design to balance computation efficiency and design flexibility; (2) a program protection mechanism is developed based on un-resemble coder algorithm to protect the program data from being read out by system intruders so as to improve the security of the sensor device; (3) an air-programming component is proposed to enable an efficient remote programming which is a very common requirement in network deployment and maintenance. In this paper, we present the design, implementation, and evaluation of the prototype sensor device based on a general configurable FPGA platform for developing next-generation sensor devices. Initial results of the first version of prototype chip will also be introduced.