## Sensor Bus Standards
### New Intramodule Sensor Bus for Low Power Microsystems  
The Intramodule Multielement Microsystem (IM\({}^{2}\)) sensor bus defines a set of specifications to provide a universal interface between a microcontroller and \"simple\" sensor nodes within a microsystem. As noted above, this intramodule bus fits within the architecture illustrated in Figure 7. In order to minimize power consumption, physical size, and cost, the \"simple\" sensor nodes will include signal conditioning and sensor bus communication hardware, but they are not required to implement the complexity of a microcontroller as required by many bus structures. Removing the necessity to have a microcontroller at each sensor node is a key element motivating the development of the IM\({}^{2}\) bus which provides an optimized interface for intramodule communication over a short physical distance. The IM\({}^{2}\) Bus is an expansion of IEEE P1451.2 TII sensor bus and is designed to have one bus controller (NCAP) and many STIMs to support distributed multidrop sensor module nodes as shown in Figure 8. This bus is also required to support sensor data in both analog and digital formats and offer power management features such as support of normally-off operation. IM\({}^{2}\) bus supports \"sleep mode\" operation and allows the sensor bus clock to be disabled resulting in a reduction of unutilized power  
Figure 8: Typical IM\({}^{2}\) bus configuration. dissipation. Finally, IM2 bus is designed to be a superset, or extension, of the TII bus which maintains hardware compatibility with TII. A description of the IM2 bus signals is given in Table 1. Although hardware compatibility with the IEEE P1451.2 standard has been maintained (i.e., all of the same signals are used with the same logic and function), the data protocol has been modified, features have been added to support plug-n-play, and an additional (optional) signal has been added. To satisfy the needs of microsystems with multiple internal sensor nodes, the data protocol has been modified since IEEE P1451.2 protocol assumes a point-to-point connection. The primary modifications from the TII bus include:  
* addition of a chip address byte at the beginning of each instruction
* implementation of two addressing modes
* compact address mode (chip address and function in one 8-bit word)
* full address mode (chip address and function in individual words)
* modification of the NSDET signal function to serve as a plug-n-play indicator
* definition of DOUT as either digital, analog, or frequency-coded data
* addition of the second power line to support power management features  
IM2 maintains both of the advantages of TII and I2C bus. It allows up to 255 smart nodes within each microsystem module of the overall system. Each smart node can be connected to the bus without affecting any circuits on the bus. It supports the plug-n-play function; the microcontroller can easily detect the new node added to the bus. Because some dedicated lines are used for the control signals, the hardware of IM2 is simple and power efficient. IM2 provides a simple but universal interface between the microcontroller and sensors, and is hardware compatible with IEEE1421.2 standard. It is best suited for the microsystems having severe power limitation and short wiring distances between the module controller and the sensor nodes. A full description of this bus structure as well as a sensor interface circuit which implements its protocols will be published in the literature soon.