OCP Interface for SoC - Verifying the Implementation of Embedded Processors

By Yasumasa Nakada, Chief Specialist, IP Development Group and Masataka Matsui, Senior Manager, SoC Research & Development Center, Toshiba Corporation Semiconductor Company

Courtesy of OCP-IP

Adopting OCP for in-house SoCs

Toshiba Semiconductor Company decided to widely adopt OCP as the host interface for in-house IP cores. OCP, which is standardized by OCP-IP, is the specification standard host interface for these cores (figure 1). Differing from existing host interfaces, in which each signal line is shared by several IPs, OCP defines the channel method by securing an effectual data transfer rate. In order to establish a design methodology that could change existing IP cores into an OCP-compliance core, the MeP host interface (the processor core for motion picture products, etc.) was changed into OCP to produce a sample SoC MeP core. MeP includes features which allow the size of the cache memory and local RAM to be changed, the ability to change the number of channels and level for interruption, and the variation of time and counter features. MeP can also be used with DSP and hardware engines (figure 2).

In the sample SoC, multiple MeP functions were used for video processing, audio processing etc. Each module is connected to others by the interconnect bus called, “MeP bus”.

Figure 1. The concept of OCP

Figure 2. The structure of MeP module

Reducing the development cost by using a general interface

The MeP bus includes a protocol from the Toshiba original specification. As a result, when the design of the modules connected to the MeP bus are changed, a re-evaluation of the whole SoC needs to be done in order to determine if the MeP bus satisfies the required performance. These re-evaluations can sometimes cause delays in the development period. By replacing the connection between the modules with OCP, the development and modification work for each module is reduced (figure 3). As a result, there are four requirements for the new host interface: (1) it must be an open standard protocol, (2) it cannot depend on any existing bus protocol, (3) it must be a configurable specification (depending on the required performance) (4) it must be an evolving standard. It is important that the protocol be an open standard in order to accommodate the procurement or licensing of an interconnect IP core from an IP vendor, such as a bus IP or cross-bar switch IP.

Although it was possible to use other industry standard specifications for the on-chip bus protocol, OCP was adopted because it is the only interface specification that does not depend on any data transmission protocol tied to CPU cores or buses. For this reason, OCP is the most suitable bus protocol for use in SoC design.

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Figure 3. The structure of the sample SoC

Defining the specification using “Taguchi methods”

OCP defines various functions, but the mandatory interface specification set by OCP is moderate. When designing a SoC with OCP-compliant host interface IP, a designer defines the detailed specification by considering the performance and cost of the SoC. The first step in converting the MeP host interface into OCP was to define the performance parameters.

During this process “Taguchi methods” were used to achieve the targeted performance and make the development process more efficient.

Securing bandwidth for the CPU

Initially, the priority conditions were listed: for example, 130-150MByte/sec bandwidth for the bus between the CPU core and synchronous DRAM at the peak. Synchronous DRAM is shared by the CPU core and each module of the SoC. When the access for synchronous DRAM from each module increases, access to DRAM from the CPU core is delayed. Since the CPU core controls the entire SoC, the operation delay also influences the entire SoC. When the OCP parameters were configured, the bandwidth was reserved by the max load assumption in order to avoid the delay of the CPU core operation at the peak. Additionally, the necessary bandwidths for each module were defined, which deferred any peak bottlenecks. To determine the simulation performance with the Taguchi methods, 12 evaluation properties were prepared (table 1). In accordance with these requirements, the most appropriate specification was defined. After this, the assignment of OCP threads was decided. In the OCP specification, if the data transmissions use the same thread, the order of the load and store is maintained. This condition implies that the exclusive access control and bus utilization rate will come down. As a result, separate threads are used for transactions which can be transmitted out-of-order.

Table 1. 12 evaluation properties

Managing performance assurance and shortening the development period

Simulations based on Taguchi methods were repeated and OCP parameter configuration specification was defined by adopting the best simulation result. As the SoC scale became larger, the number of modules embedded and the number of buses increased. As performance is refined, many parameters need to be considered since the development method described can cause an increase in development time. When using Taguchi methods for performance tuning, the load for the development is restrained to maintain enough performance. This method is useful when the number of OCP modules in a SoC are increased.