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authorRob Austein <sra@hactrn.net>2020-09-13 23:06:24 +0000
committerRob Austein <sra@hactrn.net>2020-09-13 23:06:24 +0000
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-= Comparison of On-Chip Bus Standards =
-
-== Introduction ==
-This document contains a brief summary of different on-chip bus
-standards. The standards are described and compared based on license and
-availability, technical specifications and general usage.
-
-The purpose of the document is to provide a basis for selecting the
-primary bus standard for the Cryptech Open HSM project.
-
-
-== Overview ==
-System on Chip (SoC) designs require some sort of connectivity between
-the different components (called cores or IP-cores, as in Intellectual
-Property) that are placed onto the same die.
-
-There are several standards for on-chip interconnect, standards that
-provide technical diversity that might be required by the
-system. Typical differences are:
-
- - Performance. The capacity as well as latency.
-
- - Intelligence. Simple master-slave read/write access or DMA-transfers,
- coherence support etc.
-
- - point to point or point to multipoint. Basically bus based or switch
- fabric.
-
-
-There are also non-technical differences:
-
- - Licensing and pricing. Does using a standard add monetary cost and
- does using the standard infer restrictions in sharing, disclosure of
- source code?
-
- - Market share. The market share is primarily interesting as basis for
- the availability of other cores that could be integrated.
-
-
-== Description of Standards ==
-
-=== AMBA ===
-AMBA (Advanced Microcontroller Bus Architecture) [#fn1 (1)], [#fn2 (2)] is a family of
-interconnect standards from ARM Ltd. AMBA is widely used in systems
-implemented in ASICs (for example mobile phone platforms), but are also
-used in FPGAs. AMBA is for example used by the LEON [#fn3 (3)] processor
-cores and subsystem GRLIB.
-
-AMBA currently contains four main interconnect types:
-
- - APB. A simple register read/write bus used to connect simpler
- devices such as timers, IRQ handlers, slow serial I/O such as UARTS
- and GPIO interfaces. The peripherals are connected to a common bus
- with a single master.
-
- - AHB. A more advanced bus based interconnect. Supports more complex
- data transfers of up to 1 kByte data. Supports multiple masters.
-
- - AXI. A switch fabric based interconnect that supports multiple
- parallel transfers, multiple masters etc.
-
- - ACE. A low latency interconnect that supports cache coherency to
- allow the design of multicore, multiprocessor systems on-chip.
-
-(There are also additional protocols in the AMBA specification for
-things like tracing etc.)
-
-The license model for AMBA is _Open_ according to ARM. This seems to
-mean that one can use AMBA to build a system. But at the same time, ARM
-has intellectual properties to parts of the technology as well as
-trademarks. For more information on ARM licensing, see [#fn4 (4)].
-
-The !OpenCores project [#fn7 (7)] lists several cores as well as tools for
-different AMBA interconnect types.
-
-Pros:
- - Technically advanced and covers a wide range of system
- requirements.
-
- - A huge user base.
-
- - A huge selection of third party support in terms of tools as well as
- cores. Most of these cores and tools are commercial and proprietary,
- closed source.
-
-Cons:
- - Licensing. Would Cryptech need to get a license?
-
- - Availability of open cores
-
-
-
-=== Avalon ===
-Avalon [#fn5 (5)] is a proprietary switch fabric interconnect from Altera
-corporation. It is used in systems developed using the Altera Nios-II
-[#fn6 (6)] family of soft processor cores and related peripherals.
-
-According to Altera, the license for Avalon is open: "Avalon interfaces
-are an open standard. No license or royalty is required to develop and
-sell products that use, or are based on Avalon interfaces."
-
-As far as we can discern, Avalon is not generally used outside of Altera
-based designs and not supported by a large group of third party
-vendors. The OpenCores project lists only a few cores that uses Avalon
-as interface standard.
-
-=
-Pros:
- - Good technical features.
-
- - Easy integration in Nios-II based systems.
-
-Cons:
- - Limited to Altera based FPGA designs.
-
- - Low support from open and proprietary third party suppliers of tools
- and cores.
-
-
-
-=== !CoreConnect ===
-!CoreConnect [#fn8 (8)] is an interconnect standard initially developed by
-IBM. The standard is now used by several vendors, for example the
-FPGA-vendor Xilinx[#fn9 (9)].
-
-Similarly to AMBA, !CoreConnect contains several types of buses providing
-simple peripheral access (DCR), high speed access for processor based
-systems (OPB), as well as multicore solutions (PLB).
-
-The license for !CoreConnect is granted by IBM [#fn10 (10)]. The license seems to be
-an AS IS-license, but contains a lot of other regulations. IBM holds a
-number of patents related to !CoreConnect (see the license agreement).
-
-Pros:
- - Good support on for systems implemented on Xilinx FPGAs.
-
-Cons:
- - Low support by open cores and tools.
- - License agreement.
-
-
-=== OCP ===
-The Open Core Protocol [#fn11 (11)] is a vendor neutral open interconnect standard
-being developed by the EDA standards organisation Accellera [#fn12 (12)]. The
-standards was previously developed by the vendor organisation OCP-IP [#fn13 (13)],
-but were transferred to Accellera in October 2013.
-
-Like AMBA, OCP contains a wide range of interconnect types from simple
-register read/write access over a common bus to point to
-point-interconnect and coherency support.
-
-There are quite a few commercial cores using OCP, but there seem to be
-very few open cores using OCP. !OpenCores only lists a few cores and
-they are all bridges used to connect OCP to AMBA or Wishbone.
-
-The license for accessing the specification itself is an amended AS
-IS-type license[#fn14 (14)]. The license for the interconnect seems to be rather
-open.
-
-Pros:
- - Good technical features.
-
-Cons:
- - Not very common in use by open cores.
-
-
-=== Wishbone ===
-Wishbone [#fn15 (15)][#fn16 (16)] (often written WISHBONE) is an open interconnect
-standard developed by members of the !OpenCore project as an alternative
-to commercial solutions - primarily AMBA.
-
-Wishbone supports bus based as well as switch fabric interconnect
-solutions of Wishbone cores. There are cores and tools to create CPU
-based systems with buses and fabrics. Technically Wishbone is simpler
-that AMBA and !CoreConnect, but provides multimasters, point to point
-switch fabrics, etc.
-
-There are tools available to generate Wishbone interfaces for a core as
-well as creating a Wishbone connected system with different types of
-interconnect solutions.
-
-The main use is related to the OpenRISC CPU core platform
-[#fn17 (17)][#fn18 (18)]. !OpenCores lists a huge selection of cores with Wishbone
-support. The majority of these cores have LGPL and GPL licenses. There
-are also third party commercial vendors that support Wishbone cores and
-systems.
-
-The license for the Wishbone standard is public domain and dos not
-impose any restrictions on usage in cores and systems. The
-specification document itself is close to Creative Commons CC-BY.
-
-Pros:
- - Fairly good technical support.
- - Good support from open tools and cores.
- - Public domain license.
-
-Cons:
- - Not as advanced. No good coherency support for example.
-
-
-== Conclusions ==
-OF the different standards, only two standards are really interesting
-for Cryptech - AMBA and Wishbone.
-
-From a technical point of view, selecting AMBA would be the proper
-choice. AMBA provides all types of interconnect that a Cryptech
-implementation might need. Also, building a Cryptech implementation
-using third party cores (CPU cores for example) would be easier with
-AMBA than the other standards. Wher AMBA falls short is the questions
-related to licensing as well as the a bit less common support from open
-cores and tools.
-
-Based on ease of licensing, openness and availability of open cores,
-Wishbone is an easy choice. Wishbone would quite probably meet all
-performance and functionality requirements a Cryptech implementation
-might have. Integration with and support from commercial cores, tools
-and vendors will however not be as good. Choosing Wishbone will quite
-probably mean more work for the Cryptech project to deliver cores and
-tools. And for the users of Cryptech Wishbone may also require more work
-and thus reduce the interest Cryptech as a HSM solution.
-
-
-
-== References ==
-[=#fn1 (1)] https://en.wikipedia.org/wiki/Advanced_Microcontroller_Bus_Architecture
-
-[=#fn2 (2)] http://www.arm.com/products/system-ip/amba/amba-open-specifications.php
-
-[=#fn3 (3)] https://en.wikipedia.org/wiki/LEON
-
-[=#fn4 (4)] http://www.arm.com/products/system-ip/amba/index.php?tab=AMBA+Trademark+Guidelines
-
-[=#fn5 (5)] http://www.altera.com/literature/manual/mnl_avalon_spec.pdf
-
-[=#fn6 (6)] http://www.altera.com/devices/processor/nios2/ni2-index.html
-
-[=#fn7 (7)] http://opencores.org/
-
-[=#fn8 (8)] https://en.wikipedia.org/wiki/CoreConnect
-
-[=#fn9 (9)] http://www.xilinx.com/products/intellectual-property/dr_pcentral_coreconnect.htm
-
-[=#fn10 (10)] http://www.xilinx.com/ipcenter/doc/ibm_click_core_connect_license.pdf
-
-[=#fn11 (11)] https://en.wikipedia.org/wiki/Open_Core_Protocol
-
-[=#fn12 (12)] https://en.wikipedia.org/wiki/Accellera
-
-[=#fn13 (13)] http://www.ocpip.org/
-
-[=#fn14 (14)] http://www.ocpip.org/license_signup.php
-
-[=#fn15 (15)] http://opencores.org/opencores,wishbone
-
-[=#fn16 (16)] https://en.wikipedia.org/wiki/Wishbone_(computer_bus)
-
-[=#fn17 (17)] http://openrisc.net/
-
-[=#fn18 (18)] http://opencores.org/or1k/Main_Page
-
-
-== Copyright and License ==
-
-This document has been written by Joachim Strömbergson.
-
-(c) 2014 SUNET - The Swedish University Network
-
-This document is licensed under a Creative Commons license (CC BY 3.0).
-For more information, see:
-
-https://creativecommons.org/licenses/by/3.0/