The Freescale MMA7260Q custom CompactRIO module has two main design requirements. First, with the module, the user must be able to control the inputs of the Freescale MMA7260Q sensor (selectable g-range, sleep mode, etc.) in software. Secondly, the module should output the proper values from the Freescale MMA7260Q sensor so other I/O (NI 9215 analog input module) can read the sensor values with 16-bit accuracy.
From the datasheet of the Freescale MMA7260Q, the signal connection requirements are found to be the following:
1) Inputs – g-Select1, g-Select2, and Sleep Mode
2) Outputs – Xout, Yout, and Zout
3) Power and Ground – Vss and Vdd
The inputs of the Freescale MMA7260Q are all TTL logic, and as such will be controlled by the outputs from the Xilinx Virtex-II FPGA which resides on the backplane of the CompactRIO chassis. The FPGA itself will be programmed using the LabVIEW FPGA Module, to allow for graphical-programming at a much higher level of abstraction than that of a hardware descriptive language (HDL), which is traditionally used to program an FPGA.
The outputs of the Freescale MMA7260Q will be routed to the BNC connectors of the custom module. The design decision to use BNCs versus an alternative connector such as a 25-pin combicon, is to provide the most durable and reliable connectivity of the Freescale MMA7260Q outputs to the NI 9215 Analog Input Module.
The final signal connection requirement of the Freescale MMA7260Q are the power and ground pins. While the ground pin connection is straightforward and will be tied to the ground from the CompactRIO backplane, the power requirement of the MMA7260Q varies from the available power supplied by the CompactRIO backplane. Power specifications from the CompactRIO backplane are discussed in more detail in the CompactRIO MDK. In this case, it will be necessary to incorporate power supply circuitry that will drop the supply voltage of 5 V to the required 3.3 V for the MMA7260Q. This will be accomplished by using a Low-Drop Out (LDO) Regulator. This is the only additional chip used in the circuit design of the custom module, aside from the necessary surface-mount (SMT) capacitors and resistors. When discussing power, an important consideration in the design of a custom CompactRIO module, is whether isolated circuitry is needed or not. In the case of this module, because no signals will be received from the outside world, a nonisolated design will suffice. However, in many cases it would be necessary to provide an electrical and physical barrier between the power supply of the FPGA and the I/O circuitry.