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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/