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diff --git a/content/InterconnectStandards.md b/content/InterconnectStandards.md new file mode 100644 index 0000000..a31da2c --- /dev/null +++ b/content/InterconnectStandards.md @@ -0,0 +1,323 @@ +Title: InterconnectStandards +Author: trac +Date: 2016-12-15 22:44 + +# 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) [(1)](#fn1), [(2)](#fn2) 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 [(3)](#fn3) 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 [(4)](#fn4). + +The OpenCores project [(7)](#fn7) 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 [(5)](#fn5) is a proprietary switch fabric interconnect from Altera +corporation. It is used in systems developed using the Altera Nios-II +[(6)](#fn6) 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 [(8)](#fn8) is an interconnect standard initially developed by +IBM. The standard is now used by several vendors, for example the +FPGA-vendor Xilinx[(9)](#fn9). + +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 [(10)](#fn10). 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 [(11)](#fn11) is a vendor neutral open interconnect standard +being developed by the EDA standards organisation Accellera [(12)](#fn12). The +standards was previously developed by the vendor organisation OCP-IP [(13)](#fn13), +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[(14)](#fn14). 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 [(15)](#fn15)[(16)](#fn16) (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 +[(17)](#fn17)[(18)](#fn18). 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 +1. https://en.wikipedia.org/wiki/Advanced_Microcontroller_Bus_Architecture + +2. http://www.arm.com/products/system-ip/amba/amba-open-specifications.php + +3. https://en.wikipedia.org/wiki/LEON + +4. http://www.arm.com/products/system-ip/amba/index.php?tab=AMBA+Trademark+Guidelines + +5. http://www.altera.com/literature/manual/mnl_avalon_spec.pdf + +6. http://www.altera.com/devices/processor/nios2/ni2-index.html + +7. http://opencores.org/ + +8. https://en.wikipedia.org/wiki/CoreConnect + +9. http://www.xilinx.com/products/intellectual-property/dr_pcentral_coreconnect.htm + +10. http://www.xilinx.com/ipcenter/doc/ibm_click_core_connect_license.pdf + +11. https://en.wikipedia.org/wiki/Open_Core_Protocol + +12. https://en.wikipedia.org/wiki/Accellera + +13. http://www.ocpip.org/ + +14. http://www.ocpip.org/license_signup.php + +15. http://opencores.org/opencores,wishbone + +16. https://en.wikipedia.org/wiki/Wishbone_(computer_bus) + +17. http://openrisc.net/ + +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/ |