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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/