Microprocessor Subsystem

AHB Bus Structure

Clocks

Excalibur devices implement multiple internal clock domains. The processor, dual-AHB bus structure, and SDRAM controller all have their own clock domains. Two dedicated programmable phase-locked loops (PLLs) control these clock domains: one PLL generates the clock for the AHB1 and AHB2, and the other PLL generates the clock for the SDRAM controller. The AHB1 processor bus and SDRAM bus run at speeds up to 200 MHz (with fAHB2 = fAHB1/2) and 133 MHz, respectively. The clock domains are independent and adhere to the APEX^™ PLL specification and clock signals.

FPGA Configuration Logic

Excalibur devices may be configured several ways. The processor can boot independently from the FPGA, and it contains the configuration logic to program the FPGA from data stored either in external (flash) memory or downloaded into on-chip SRAM. Unlike FPGA solutions, Excalibur devices can be reconfigured at any time via processor control, while the processor continues to run. This can be done via a setup sequence, or via auto-detection when configuration is required.

In addition, the processor memory spaces can be reconfigured using traditional FPGA configuration modes and memory. In this event, the processor is held in reset, then allowed to boot only when the FPGA configuration is complete.

Microprocessor Subsystem to FPGA Interfaces

The value of integrating a processor and FPGA increases substantially with the incorporation of multiple high-speed data exchange interfaces. The Excalibur processor has two AHB bridges on AHB2 that allow the processor (or other master within the subsystem) and the FPGA to each act as bus masters on the AHB2 bus. By functioning as bus master, the master can initiate a data transfer through the bus bridge in either direction. Moreover, allowing the FPGA and the processor to initiate bus transfers enables the creation of complex systems that provide real-time interaction, between the processor and FPGA, to form complete system-on-a-programmable-chip (SOPC) designs.

The dual-port SRAM memory is accessible from both the FPGA and the processor. Data can be written to and read from this memory space, providing a simple shared data area for applications interfaces. One such application is for the processor to interface to a digital signal processing (DSP) function implemented in the FPGA by reading and writing the appropriate memory spaces within the dual-port memory.

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