Technical Insights
PCIe and RapidIO Work Together
By Charles Frazier, Mercury Computer Systems, Inc.
www.mc.com
System designers are faced with many challenging problems to solve and therefore, need a variety of technologies to best architect their solutions. In the world of interconnects, the prolific PCI Express connectivity technology works well for small systems and connecting I/O devices, while RapidIO technology is designed for low latency, high performance, and highly deterministic operation. Combining these two technologies can enhance system design and make solutions significantly better than if only one of the technologies is used.
PCI Express at the I/O
PCI Express was designed for the desktop computer market and thus its architecture works well in a system that incorporates one master and a few slave I/O devices. The proliferation of PCI Express has created many chipsets that connect to standard I/O technologies, such as Ethernet, SATA, Fibre Channel, and many others, making PCI Express ideally suited to extend to standard interconnects. But scaling of PCI Express systems to large processor systems that work on real-time events is limited due to its inherent non-real-time design.
RapidIO as the system fabric
In many applications, processing requirements have grown beyond what individual processors can deliver. This performance mismatch has created the need for a multi-peer fabric that enables the distribution of processing among many processing elements. These requirements for system connectivity and features such as flow control, data streaming, multicast, and high reliability were the driving force behind the creation of RapidIO.
The Complete System Solution
Many embedded systems have used PCI Express based components because of the large number of standard I/O interfaces and processors that it supports. A problem occurs, however, if the computational task requires scaling the system to many processors. PCI Express is not well suited to address this problem. Many embedded systems require robust I/O connectivity and very high processing rates. The combination of PCI Express enabled processors, which have robust I/O connectivity, and RapidIO fabric can provide a solution to the challenges of these demanding embedded applications.
RapidIO technology is extremely flexible and is designed to meet the stringent requirements of redundancy imposed by such industries as communications, which require topologies like dual star, mesh, and other complex designs. On the other end of the spectrum, RapidIO topologies can be configured in a daisy-chain fashion to reduce costs and meet the needs of pipelined processing systems. Figure 1 shows a typical small PCIe processor-based system using RapidIO as the interconnect technology. The PCIe-to-RapidIO bridge performs many functions, such as address translation, memory protection, multithreaded DMA, and a host of other functions. Bridges from such vendors as Mercury Computer Systems offload a significant amount of data-moving tasks from the processor and operate at line rate.
Many embedded systems require synchronization among the various processing elements. This requirement has been designed into the RapidIO specification in a variety of ways.
- Atomic operations - Enable synchronization
- Response-based transactions - When packets are received at a destination, a response packet can be sent back acknowledging receipt
- Message passing (hardware-based) - Enables data to be shared among various processors, where management of ownership is provided
- Multicast TOD event - Enables time synchronization in the <1 ?s range among processors in a system using in-band communications
Complementary technologies
RapidIO was designed and architected from the ground up as a multiprocessor fabric to meet the demanding needs of the multiprocessor environment. PCI Express was designed to enable I/O expansion from a single processor and has excellent bandwidth, low cost, I/O connectivity, and a software infrastructure to enable turnkey I/O connectivity. The complementary technologies of a serial RapidIO based fabric and PCI Express I/O connectivity enable designers to create systems that can take advantage of the strengths of these two interconnects for better overall performance.
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