The ability to use radiation-tolerant programmable digital logic parts in hardware intended for use in high-reliability space missions provides significant cost savings. The savings result partly from shorter design cycles and lower risk, but mostly from reduced parts acquisition and flight qualification costs. Without these parts, many projects would be unable to afford custom gate array implementations, and would be forced to resort to discrete component designs with the accompanying increase of system size, mass, and power consumption.
A simliar situation exists in the mixed-signal field today. Almost all spacecraft contain numerous moderate-performance analog and digital processing and I/O circuits. These circuits, used for applications such as status monitoring, motor and temperature control, and signal conditioning and processing, are widely distributed throughout the spacecraft. Traditionally, these mixed-signal circuits have been implemented almost exclusively using discrete components, because the cost savings are not significant enough to justify a custom ASIC design. However, the resources used by such circuits, including mass, power, and volume, add up very quickly. The ability to implement such designs using general-purpose, programmable analog or mixed-signal arrays would be advantageous in all respects, analogous to the advantages gained using a Field-Programmable Gate Array (FPGA) for digital circuits: lower parts acquisition and qualification costs, higher levels of integration, lower power consumption, fast turn-around time for design cycles, and improved reliability.
The Field-Programmable Analog Array (FPAA) is the analog equivalent of the FPGA, a digital programmable device such as those made by companies such as Actel, Xilinx, and Altera. Unlike FPGAs, which contain a large number of modules and interconnections allowing arbitrary configurations of combinatorial and sequential logic, FPAA devices typically contain a small number of CABs (Configurable Analog Blocks). The resources of each CAB varies widely between different commercially available and research devices. FPAAs directed toward standard analog design typically feature a CAB containing an operational amplifer, programmable capacitor arrays (PCAs), and either programmable resistor arrays for continuous-time circuits or configurable switches for switched-capacitor circuits .
A Field-Programmable Mixed-signal Array (FPMA) is comprised of digital, analog, and interface modules. In many ways, an FPMA is a more useful device than the FPAA is by itself, due to the extensive use of digital for communications throughout the spacecraft and between the spacecraft and Earth. Almost all analog circuits on board the spacecraft require interfacing to digital systems, and again there are numerous advantages to locating the interface circuitry within a field-programmable part. The digital portion of an FPMA can be relatively small and allows simple digital processing to take place close to the analog signal source without requiring a separate FPGA for the purpose.
Last updated: October 13, 1999 at 3:00pm