How to interface FPGAs to microcontrollers

By Rocendo Bracamontes Del Toro, Atmel
pldesignline.com (July 30, 2008)

As many as half of all embedded designs have an FPGA next to a microcontroller. The FPGA can be used to implement anything from glue logic, to custom IP, to accelerators for computationally intensive algorithms. By taking on some of the processing tasks, FPGAs help to improve system performance, thereby freeing up the MCU from cycle-intensive tasks. FPGAs also provide excellent performance characteristics and lots of flexibility to accommodate changing standards.

There are two basic implementations of MCU-plus-FPGA designs: putting a soft MCU core into the FPGA logic structure or using a standard product MCU with a discrete FPGA. Putting a soft core into the FPGA can be effective, but it can also be an expensive and power-hungry way to implement a microcontroller when compared to a standard product. This is especially true when using a 32-bit ARM-based core. As a result, only about one-third of FPGA-plus-MCU designs are implemented with an MCU core inside the FPGA logic. The remaining two-thirds consist of a standard product microcontroller next to a discrete FPGA.

Neither standard product microcontrollers nor FPGAs were developed to communicate with each other efficiently. They even use different languages. Thus, interfacing the two can be a challenge. FPGAs do not have any dedicated logic that communicates with microcontrollers. This logic module must be designed from scratch. Second, the communication between the microcontroller and FPGA is asynchronous. Special care is needed to resynchronize the MCU to the FPGA clock domain. Finally, there is an issue of bottlenecks, both at the interface and on the MCU bus. Transferring information between the MCU and the FPGA usually requires cycles on the MCU bus and usually ties up the resource (PIO or EBI) used to effect the transfer. Care must be taken to avoid bottlenecks with external SRAM or Flash and on the MCU bus.

There are basically three hardware options for interfacing the FPGA to the MCU: programmable I/O (PIO); external bus interface (EBI), if available; and, finally, a dedicated interface between built into the MCU between the advanced high-speed bus (AHB) and the FPGA. Which approach to use depends on the end application and the desired result.

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