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\def\deftitle{HIBI v.3 \\Reference Manual}

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\lhead{\bfseries Department of Computer Systems\\

  Faculty of Computing and Electrical Engineering}

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\textsc{Faculty of Computing and Electrical Engineering}\\[1.0cm]

\textsc{Department of Computer Systems}\\[1.0cm]

%\textsc{\LARGE Tampere University of Technology}\\[1.0cm]

%\textsc{\Large Faculty of Computing and Electrical Engineering}\\[1.0cm]

%\textsc{\Large Department of Computer Systems}\\[1.0cm]

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{ \huge \bfseries Heterogenerous IP Block Interconnection (HIBI) \\ version 3 \\ [0.5cm]Reference Manual}

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\begin{minipage}{0.4\textwidth}

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\emph{Author:}\\

Erno Salminen, \\Timo Hämäläinen

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\emph{Updated:} \\

\today

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%\title{HIBI data sheet - September 2011}

%\author{Erno Salminen}

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\section{Introduction}

\label{ch:hibi}

This data sheet presents the third version of \textit{Heterogeneous IP

  Block Interconnection} (HIBI).  HIBI is intended for integrating

coarse-grain components such as intellectual property blocks that have

size of thousands of gates, see \cite{salminen04} for examples.

Topology, arbitration and data transfers are presented first. After

that, data buffering and the structure of wrapper component are

discussed. Finally, the developed runtime configuration is presented

followed by comparison to the previous version of HIBI.

HIBI is a communication network designed for System-on-Chips. It can

be used both in FPGA and ASIC designs (field-programmable gate-array,

application-specific integrated circuit). Fig.~\ref{fig:soc_concept}

shows an example SoC at conceptual level. There are many different

types of IP blocks (intellectual property), namely CPU (central

processing unit) for executing software, memories and IP blocks that

are either fixed function accelerators or interfaces to external

components. All these are connected using an on-chip network.

\begin{figure} [b]

  \begin{center}

    {\includegraphics[width=0.60\textwidth]{../Fig/Eps/fig_soc_concept.eps}}

    \caption{Conceptual structure of system-on-chip}

    \label{fig:soc_concept}

  \end{center}

\end{figure}

\subsection{Main points}

The major design choices for HIBI were

\begin{itemize}

\item   IP-block granularity for functional units

\item   Application independent interface to allow re-use of processors and IP-blocks

\item   Communication and computation separated

\item   Communication network used in all transfers, no ad-hoc wires between IPs

\item   support local clock domains for IP granularity

\end{itemize}

A parameterizable HW component, called HIBI wrapper, is used to

construct modular, hierarchical bus structures with distributed

arbitration and multiple clock domains as shown in Fig

\ref{fig:hierarchy} (explained later in detail).  This simplifies

design and allows reuse since the same wrapper can always be utilized.

Configuration takes place both at synthesis time (e.g. data width and

buffer sizes) and on runtime (arbitration parameters).

In addition, since we are targeting also FPGAs, there are some additional constraints

\begin{itemize}

\item   keep the number of wires low - to avoid exhausting routing resources

\item   avoid global connections - to avoid long combinatorial routing delays

\item   avoid 3-state wires - to simplify testing and synthesis (most FPGAs allow three-state logic onlu in I/O pins)

\end{itemize}

\subsection{Versions}

The development of HIBI \cite{kuusilinna98, lahtinen02, lahtinen04,

  salminen10} started in 1997 in Tampere University of Technology.

Currently, there are 3 versions of HIBI, denoted as v1-v.3. However,

certain basics have remained unchanged. Hence, in the remainder the

version number is omitted unless, it is necessary.

In version 2, the biggest changes were removing tri-state logic and

increasing modularity and configurability.

For version 3, address decoder logic was modified to simplify

usage. Furthermore, the tx and rx state machines were re-factored,

which also necessitated minor change in bus timing. These latter FSM

changes do not affect the IP, though.

\section {HIBI topology}

\begin{figure*}

  \begin{center}

    {\includegraphics[width=0.85\textwidth]{../Fig/Eps/fig_topo_hibi_hierarchy.eps}}

    \caption{Example of a hierarchical HIBI network with multiple clock domains and bus segments}

    \label{fig:hierarchy}

  \end{center}

\end{figure*}

The topology in HIBI is not fixed, but configurable by the

designer. HIBI network consists of wrappers, bus segments, and

bridges. These are the basic building blocks from which the whole

network is constructed and configured.  All wrappers in the system are

instantiated from the same parameterizable HDL (HW description

language) entity and bridges are constructed by connecting two

wrappers together.  If the connected segments use different data

widths, the bridges are responsible for the data width adaptation.

All wrappers can act both as a \textit{master} and a

\textit{slave}. Masters can initiate transfers and slaves can only

respond. In many buses, most units operate in on mode only and only

few in both modes. In the most simple case, there is only segment and

the topology is hence single shared bus. However, HIBI network can

have multiple segments which form a hierarchical bus

structure. Segments are connected together using bridges. Bridges

increase latency but, on the other hand, hierarchical structure allows

multiple parallel transactions. Bridge are simply constructed from 2

wrappers.

For the IP, the wrapper offers FIFO-based (first in, first out)

interface, as depicted in Fig.  In network side, all signals inside a

segment are shared between wrappers and no dedicated point-to-point

signals are used. Arbitration decides which wrapper (or bridge)

controls the segment and the utilized arbitration algorithms

distributed to wrappers without any central controller.

\subsection{Example of hierarchical topology}

Bus performance can be scaled up by using bridges. Segments having

only simple peripheral devices can have a slow and narrow bus while

the main processing parts have higher capacity buses.

Fig.~\ref{fig:hierarchy} depicts an irregular HIBI network. The

example has a point-to-point link ($Seg A$), hierarchical bus ($Seg B$

and $SegC$), and multibus topology ($Seg C$ and $SegD$). Furthermore,

$Seg B$ is wider than other segments and thus offers greater

bandwidth.  In the multibus configuration, each IP must decide which

bus to use while sending. Note that $Seg A$ could be implemented

without wrappers since there is no need for arbitration.

The example shows four clock domains. Agents in $Seg A$ and $SegB$ are

inside one domain and HIBI wrappers on $Seg C$ are in one domain.

However, two IPs in the top right corner use different clock than the

wrappers of $Seg C$.  The IPs in the bottom right corner and all

wrappers in $Seg D$ are in one domain.  The number of clock domains is

not otherwise restricted but all wrappers in one bus segment must use

the same clock. Handshaking between the clock domains is done in the

IP-wrapper interface or inside the bridge \cite{kulmala06b,

  kulmala06e}. This allows the construction of GALS systems. The

example shows only one bridge but HIBI does not restrict either the

number of bridges or hierarchy levels in contrast to many bus

architectures.

\subsection{Switching}

Transfers inside a bus segment are circuit-switched and use a common

clock due to (current) implementation of the distributed arbitration.

However, HIBI bridges utilize switching principle that resembles

packet-switching so that bus segments are not circuit-switched

together.  Instead, the data is stored inside the bridge until it gets

an access to the other segment. The data is forwarded to next segment

as soon as possible like in wormhole routing. However, no guarantees

are given for the minimum length of continuous transfer.  If the

bridge cannot buffer all the data, the transfer is interrupted and the

source segment is free for other transfers.  The interrupted wrapper

will continue the transfer on its next turn.  It is also possible that

a bridge buffers parts from multiple transfers.

\section {Data transfer operations}

In HIBI, all transfers are bursts. In practice, there is always 1

address word followed by n data words. The max. n is wrapper-specific

arbitration parameters. HIBI v2. used multiplexed address and data

lines, but HIBI v.3 allows transmitting them in parallel. Due to

multiplexed addr/data lines, it is beneficial to send many data into

single address. This is quite different from ``traditional'' memory

accesses, with address and data at the same time. Hence, the

destination IP should keep track of received data count, e.g. TUT's

SDRAM controller can do this to avoid excess transmitting addr + data

pairs

The transfers are pipelined with arbitration, and hence the next

transfer can start immediately when the previous ends. The protocol on

the bus side is optimized so that there no wait cycles are allowed

during a transfer. This means that is sender runs out of data or the

receiver does not accept it fast enough, the transfer is

interrupted. On the next arbitration turn, the wrapper it continues

automatically. Note that IP may transfer data at pace it wishes. IP

has only to ensure that there is space in TX FIFO while writing and

that RX FIFO is not empty while reading.

In order to increase bus utilization, HIBI uses so called

split-transactions in read operation. It means that single read

operation is split into two phases: request and response. The bus

segment is released while the addressed IP handles the read request

and prepares its response. The other wrappers may use bus during that

period and this increases the overall performance, although a single

read becomes a little slower due additional arbitration round.

\begin{figure}

  \begin{center}

    {\includegraphics[width=0.7\textwidth]{../Fig/Eps/fig_basic_tx.eps}}

    \caption{Example of read and write operations.}

    \label{fig:basic_tx}

  \end{center}

\end{figure}

\begin{figure}

  \begin{center}

    {\includegraphics[width=0.4\textwidth]{../Fig/Eps/fig_basic_tx2.eps}}

    \caption{Basic transactions are write and read.}

    \label{fig:basic_tx2}

  \end{center}

\end{figure}

Write operation

\begin{itemize}

\item   Includes destination address

\item   Data is sent in words (=HIBI bus width)

\item   Several words can follow: all will be sent to the same destination address

\end{itemize}

Read operation

\begin{itemize}

\item   Includes exactly two words: destination address and return address (where to put the data)

\item   Data is received in words

\item   Several words can be received (all to same return address)

\begin{itemize}

\item   No handshaking: data is transmitted/received when bus, sender, or receiver are available

\item   No acknowledgements or flow control

\end{itemize}

\end{itemize}

Figs.~\ref{fig:basic_tx} and~\ref{fig:basic_tx2} depict the two basic

transfers: sending the read request, write, and the response to

read. IP can send multiple read requests before the previous ones have

completed. It is the responsibility of the requestor to keep track

which response belongs to which request. This can be implemented with

appropriate use of return addresses. The reader does not get data any

faster but the advantage is that the shared medium is available for

other agents in the middle of the transmission process and

consequently the achieved total throughput increases. In

packet-switched networks the split-transactions are commonly used and

also in modern bus protocols, such as AMBA

Since there is exactly one path between each source and destination,

all data is guaranteed to arrive in-order and hence no reordering

buffers are needed at the receiver.  Data can be sent with different

relative priorities. High priority data, such as control messages,

bypass the normal data transfers inside the wrappers and bridges

resulting in smaller latency. This does not change the timing of bus

reservations, but it selects what is transferred first.

\subsection{HIBI Basic Transaction Motivation}

HIBI was motivated by streaming applications where continuous flow of

data is transmitted between IPs. Destinations are merely ports than

random accessed memory locations. Hence, HIBI is not natively a

processor memory bus but can be used for it as well.

HIBI does not implement end-to-end flow control but the IPs must do

not explicitly. The FIFO buffers and rx and tx side may get full if

the receiver does not eject data fast enough, and this will throttle

the transmitter as well. The wrappers takes care of retransmission at

the link level. (HIBI v.1 dropped data if the receiving buffer got

full but usage of v.1 is not recommended anymore).

\begin{figure*}

  \begin{center}

    {\includegraphics[width=0.8\textwidth]{../Fig/Eps/fig_tx_steps.eps}}

    \caption{Logical steps that IP does during transaction.}

    \label{fig:tx_steps}

  \end{center}

\end{figure*}

Fig.~\ref{fig:tx_steps} shows the steps that IP needs to take when

communicating using HIBI. On the left, IP sends data when the TX FIFO

is not full. It must assign data, address valid (strobe), command, and

write enable signals at the same time. When receiving data, IP first

checks is the incoming value address or data word. This is done by

examining the address valid signal. One word is removed from the FIFO

on every clock cycle when receiver assigns read enable signal. Next,

IP must check is the operation write or read. In case of write, it

stores the incoming data to location defined by the address. In case

of read, the second word denotes the return address. It is the

address, where the read data word must be transmitted.

\section{Addressing}

All IP-blocks have unique address and register space defined at design

time and every transfer starts with single destination address.

Source identification not included in basic transfer and hence

a)      Use data payload to define source, e.g. first world in a data packet

b) Use unique address inside IP block for each source (IP knows from

the destination address the sender)

Every wrappers has a set of addresses and they set with a VHDL generic

(automatic by Kactus).  Wrappers may have varying address space sizes,

e.g. simple UART has only 2 addresses whereas memory has 16K

addresses. Incoming Addresses go through the receiving wrapper to the

receiving IP and it can identify the incoming data by its address. For

example, the uppermost bits define which IP is addressed and the

lowermost define the register of that IP.

There are wo ways to set addresses

1. manually

2. A generator script in Kactus tool does this automatically according

to system specification

IP may write arbitrarily long bursts to wrapper. Perhaps only one

address in the beginning followed by arbitrary number of data

words. Moreover, IP writes data in arbitrary pace to wrapper. There

can be any number of idle cycles between data words. Therefore, the

bursts sent by the IP do not necessarily have the same length in the

bus (between wrapper). For example, wrapper may split long IP-transfer

into multiple bus transfers if the arbitration algorithms gives

ownership to another wrapper in the middle. Each part of the transfer

starts with the same address as previous.  On the other hand, a

wrapper may send many short IP-transfers consecutively at one turn.

These properties have two consequences:

1. Bursts from multiple source IP will be interleaved

2. Destination may get different number of addresses than sender.

Note that the destination IP does not know the sender unless it is

separately encoded into data or address

\subsection{HIBI destination addresses and channels}

In HIBI v.2, all transfers are bursts, i.e. address is transmitted

only in the beginning of the transfer and it is followed by one or

more data words.  The maximum burst length is wrapper-specific. HIBI

uses mainly two-level addressing scheme: the upper bits of the address

identify the target terminal (e.g.  $destination_0$) whereas the lower

bits define the additional identifier. This identifier can be used

either as an address to local memory, to select the correct reception

channel on DMA, to identify the source of the data, or to select

requested service. Certain packet-switched networks (at least those

implemented in this work) allow only one address per terminal. In that

case, the second level address must increase the header length.

HIBI destination addresses are

1. internal registers

2. ports (to/from IPs internal logic)

3. IPs memory locations transparent to outside

Burst transfers use channels (or ports) and IP block must perform

addressing (increment) internally since all data is sent to one

address. If IP's memory is transparent, the address seen outside

includes also IP-block address (e.g. in address 0xB100, oxB000 defines

the target IP and 0x100 internal memory)

\begin{figure}

  \begin{center}

    {\includegraphics[width=0.5\textwidth]{../Fig/Eps/fig_chan_addr.eps}}

    \caption{Relation between addresses and channels.}

    \label{fig:chan_addr}

  \end{center}

\end{figure}

HIBI transfers can be abstracted as channels at IP-block side (but not

formally specified how). Easiest way to separate channels is to use

unique HIBI addresses. It is IP/System level design issue is to give

meaning to the channels. For example, accelerator receives data from

CPU0 via channel 0 and from CPU1 via channel 1 and so on. Basic HIBI

transactions are used to handle possible flow control and handshaking

in addition to transfers. Fig.~\ref{fig:chan_addr} shows an example

with 6 channels (addressing style of HIBI v.2) .

Note that all incoming channels 4-6 have the same 4 upper bits in

their addresses. In other words, the example uses a convention that

the base address of IP1 is 0xC00 and therefore its uppermost address

is implcitly 0xCFF. The channels can be easily distinguished from the

lowest address bits. In HIBI v.3 the addressing defined using two

parameters: start and end address. Designer can use the same addresses

as in HIBI v.2 based systems, but this scheme allows more freedom is

address definitions, which especially beneficial in hierarchical

systems

\subsection{Implementing flow control}

Flow control and handshaking must be implemented in IP-blocks. In practise leads to IP-block specific methods which must be carefully specified at design time.

Minimum issues to be agreed

\begin{enumerate}

\item   Sender identification (e.g. unique channel address ties Ip block and purpose together)

\item Transfer size

\item Size unit in addressing(bytes/words)

\item Are byte enables utilized

\item Messages for non-posted transactions (Acknowledgements to

  write/read)

\end{enumerate}

\subsection{Example: Overlapping and breaking transfers}

It was noted that the transfers may split due to arbitration.  Example

in Fig.~\ref{fig:addr_interleaving} clarifies the phenomenon. Let us

assume that IP 1 and IP 2 send data to IP 3. We notice that IP 1 gets

the first turn in the bus its two first data words arrive to IP

3. However, after that IP 3 gets two consecutive words from IP 2, then

from IP 1 and so on. Note that in realistic case, the arbitration

happens less frequently but the example highlights the issue.

\begin{figure}

  \begin{center}

    {\includegraphics[width=0.5\textwidth]{../Fig/Eps/fig_addr_interleaving.eps}}

    \caption{The transfers may get intereleaved due to arbitration.}

    \label{fig:addr_interleaving}

  \end{center}

\end{figure}

\begin{figure*}

  \begin{center}

    {\includegraphics[width=0.65\textwidth]{../Fig/Eps/fig_hibi_wrapper.eps}}

    \caption{Structure of HIBI v.2 wrapper and configuration memory}

    \label{fig:wrapper}

  \end{center}

\end{figure*}

As a conclusion

\begin{enumerate}

\item Data is transferred in order through FIFO

\item If tx is interrupted in bus, wrapper re-sends address and

  continues tx of rest of data to destination

\item Sender tx FIFO can not be cleared once written

\item Receiver can identify to which channel data is coming based on

  address

 \end{enumerate}

\section{Wrapper structure}

HIBI network is constructed using parameterizable builgin blocks

called wrappers. The wrappers take care of arbitration, link-level

transmission, data buffering, and optional clock-domain crossing. All

signals on both sides of the wrapper are unidirectional. For example,

there are separate multibit signals data\_in and data\_out. Let us

first consider the bus side, i.e. the signals between wrappers.

The structure of the HIBI v.2 wrapper is depicted in Fig

\ref{fig:wrapper}.  The modular wrapper structure can be tuned to

better meet the application requirements by using different versions

of the internal units or leaving out properties that are not needed in

a particular application.

On IP side, there can be separate interfaces for every data priority

or they can be multiplexed into one interface.  Furthermore, the power

control signals can be routed out of the wrapper if the IP block can

utilize them.

The main parts are buffers for transferring and receiving data and the

corresponding controllers. The transfer controller takes care of

distributed arbitration. The configuration memory stores the

arbitration parameters. Relative data priority is implemented by

adding extra FIFOs. A (de)multiplexer is placed between the FIFOs and

the corresponding controller so that the controller operates only on a

single FIFO interface.  The separate (de)multiplexer allows adding

FIFOs to support priorities in excess of two without changing the

control. Currently, transmit multiplexer uses pre-emptive scheduling.

HIBI v.2 has multiplexed address and data lines whereas HIBI v.1 uses

separate address and data lines. Multiplexing decreases implementation

area because signal lines are removed and less buffering capacity is

needed for the addresses.  This causes overhead in control logic but

that is less than the saving in buffering.  Having fewer wires allows

wider spacing between wires and hence lower coupling capacitance.  On

the other hand, the saved wiring area can be used for wider data

transfers to increase the available bandwidth.  The HIBI protocol does

not require any specific control signals, but message-passing is

utilized when needed. HIBI v.1 assumes strictly non-blocking transfers

and omits handshake signals to minimize transfer latency but one

handshake signal \textit{Full} was added to HIBI v.2 to avoid FIFO

overflow at the receiver.  As a result, blocking models of computation

can be used in system design and, in addition, the depths of FIFOs can

be considerably smaller than in HIBI v.1.

\subsection{Bus-side signals}

All outputs from wrappers are ``ORed'' together and OR-gates' outputs

are connected to all wrappers' inputs. This scheme avoids the

tri-state logic that was used in HIBI v.1.

Table~\ref{table:bus_signals} lists the bus side signals and

Fig.~\ref{fig:3_wrappers} illustrates the connection between wrapper

and OR-gates. The cycle-accurate bus timing is omitted from this used

guide for brevity. All bus side outputs come directly from register

except the handshaking signal full.

\begin{table*}

  \caption {The signals at bus side, i.e. between the wrappers, in

  \label{table:bus_signals}

    v.2  and v.3 }

  \begin{center}

    \begin{tabular}{l | l | l | l}

      \hline

      Signal & Width   & Dir. & Meaning \\

      \hline \hline

      data   & generic & i+o  & Data and address are multiplexed into single set of wires \\

      av     & 1       & i+o  & Address valid. Notifies when address is transmitted \\

      cmd    & 3       & i+o  & Command: read or write, data or conficuration etc. \\

      full   & 1       & i+o  & Target wrapper is full and acannot accept the data. Current transfer will be repeated later \\

      lock   & 1       & i+o  & Bus is reserved \\

      \hline

    \end{tabular}

  \end{center}

\end{table*}

\begin{figure*}

  \begin{center}

    {\includegraphics[width=0.65\textwidth]{../Fig/Eps/fig_hibi_3_wrappers.eps}}

    \caption{Structure of HIBI v.2 wrapper and configuration memory}

    \label{fig:3_wrappers}

  \end{center}

\end{figure*}

The number of data bits can be freely chosen.  This is beneficial, for

example, when error correcting or detecting codes are added to data

and the resulting total data width is not equal to any power of two.

Active master asserts $Lock$ signal when it reserves the bus.

Handshaking is done with the $Full$ signal. When $Full$ is asserted,

the data word on the bus must be retransmitted by the wrapper.  To

improve modularity, all signals are shared by all wrappers within a

segment and no point-to-point signaling is required. Consequently, the

interface of a wrapper does not depend on the number of agents and the

wrapper can be reused more easily.  An OR network was selected for bus

signal resolution.

The HIBI implementation pays special attention on minimizing the

transfer latency by removing empty cycles from the arbitration process

by pipelining.  Empty cycles are here defined as cycles when at least

one wrapper has data to send but the bus segment is not reserved.  An

optimized protocol allows lower frequency, and hence lower power, for

certain performance level than inefficient protocol.  Empty cycles

appear also when bus utilization is low as distributed round-robin

arbitration takes one cycle per agent. If only one agent is

transmitting, it has to wait a whole round-robin cycle between

transfers. In such cases, the priority-based arbitration is useful.

\subsection{IP-side signals}

The signals at IP interface are mostly the same signals as in the bus side. Interface signals are connected to FIFO buffers inside the wrapper  and all output signals of the wrapper come from registers.

Most signals are driven by both IP and wrapper

\begin{itemize}

\item   Command

\item   Address / Address valid

\item   Data

\begin{itemize}

\item   May have high (message) and low (data) priotities (depends on wrapper type)

\item   Priority is defined by transmissting IP-block (source)

\end{itemize}

\end{itemize}

On the other hand, the FIFO access control signals depend on the

direction. Both control signals Write enable and Read enable and

driven by wrapper. The status signals are driven by wrapper. There are

always at least two status signals FIFO full and FIFO empty. In

addition, the FIFO buffers developed for HIBI offer two others: One

data left at FIFO and One place left at FIFO, which may simplify the

logic IP.

The address signals at IP side offer few choices that described next.

\begin{figure*}

  \begin{center}

    {\includegraphics[width=0.6\textwidth]{../Fig/Eps/fig_ip_signals.eps}}

    \caption{The signals between IP and wrapper}

    \label{fig:ip_signals}

  \end{center}

\end{figure*}

Fig~\ref{fig:ip_signals} depicts the signals between IP and wrapper and

Table~\ref{table:ip_signals} list their details.

\begin{table*}

  \caption {The signals at wrapper's IP interface}

  \label{table:ip_signals}

  \begin{center}

    \begin{tabular}{l | l | l | l}

      \hline

      Signal & Width   & Dir. & Meaning \\

      \hline \hline

      rst\_n & 1       & i    & Active low reset \\

      clk    & 1       & i    & Clock, active on rising edge. Same for all wrappers inside one segment \\

      data   & generic & i+o  & Data and address are multiplexed into single set of wires \\

      av     & 1       & i+o  & Address valid. Notifies when address is transmitted \\

      cmd    & 3       & i+o  & Command: read or write, data or conficuration etc. \\

      re     & 1       & i    & Read enable. Wrapper can remove the first data from FIFO \\

      we     & 1       & i    & Write enable. Adds the data from IP to TX FIFO \\

      full   & 1       & o    & TX FIFO is full \\

      empty  & 1       & o    & RX FIFO is empty \\

      one\_p & 1       & o    & TX FIFO has one place left, i.e. almost full \\

      one\_d & 1       & o    & RX FIFO has one data left, i.e. almost empty \\

      \hline

    \end{tabular}

  \end{center}

\end{table*}

\subsection{Variants of IP interface}

There are 4 variants of the IP interface depending on how to handle

a) high/low priority data: one or two interfaces

b) address and data: separate interfaces or one multiplexed

The different wrapper are denoted with postfix $\_r<x>$

r1: a) 2 interfaces  hi+lo;     b) muxed     a/d

r2: a) 1 interface   hi/lo;     b) separate  a+d

r3: a) 2 interfaces  hi+lo;     b) separate  a+d

r4: a) 1 interface   hi/lo;     b) muxed     a/d

Since these options affect only the IP side, different wrapper types

can co-exist in the same system, and the wrappers' bus side interface

is always the same. Furthermore, the addresses work directly between

wrapper types. However, hi-priority data cannot bypass lo-prior data

in wrapper types r2 and r4. However, all data is always transmitted

For example, Nios subsystems utilize commonly r4 but SDRAM utilizes

r3. This is because SDRAM ctrl distinguishes DMA configuration and

memory data traffic with priority of incoming data. It also prevents

dead-lock. Fig ~\ref{fig:ip_interface_variants} depicts variants of

wrapper's IP side signals. Interface type r1 is the ``native''

interface that is used inside all other variants.

\begin{figure*}

  \begin{center}

    {\includegraphics[width=0.8\textwidth]{../Fig/Eps/fig_ip_interface_variants.eps}}

    \caption{There are 4 variants of IP interface. There are two

      selectable features, namely separations of hi/lo-prior data and

      separate/multiplexed addressing.}

    \label{fig:ip_interface_variants}

  \end{center}

\end{figure*}

\subsection{Signal naming in VHDL}

The side and direction are marked into signal name in HIBI wrapper VHDL, for example

\begin{enumerate}

\item agent\_data\_in, agent\_data\_out,

\item bus\_data\_in, bus\_data\_out

\end{enumerate}

Fig.~\ref{fig:sgn_naming} clarifies the naming scheme.

\begin{figure*}

  \begin{center}

    {\includegraphics[width=0.5\textwidth]{../Fig/Eps/fig_sgn_naming.eps}}

    \caption{The naming convention of ports}

    \label{fig:sgn_naming}

  \end{center}

\end{figure*}

\subsection{Cycle-accurate timing}

For brevity, only the IP side timing is explained. It is actually very simple.

The timing when transmitting is depicted in Fig

1)      IP checks that tx FIFO is not full

2)      IP sets data, command, addr/av, and write\_enable=1 for one clk cycle

\begin{figure*}

  \begin{center}

    \subfigure[IP sends.]{\includegraphics[width=0.95\textwidth]{../Fig/Eps/fig_tx_timing.eps}

      \label{subfig:tx_timing}}

    \subfigure[IP receives data]{\includegraphics[width=0.95\textwidth]{../Fig/Eps/fig_rx_timing.eps}

      \label{subfig:rx_timing}}

    \caption{Examples of timing at IP interface.}

    \label{fig:interface_timing}

  \end{center}

\end{figure*}

The timing when receiving is depicted in Fig

1)      IP checks that rx FIFO is not empty

2)      IP captures data, command, and addr/av

3)      IP sets read\_enable=1 for one clk cycle

Notes on signal timing

\begin{enumerate}

\item   Very easy to write/read on every other cycle

\item Almost as easy to write/read on every cycle. Needs a bit more

  care with checking empty and full

\item IP may keep we=1 and re=1 continuously and just change/store

  data according to full/empty

\item Signal FIFO full comes from register. It goes high on the next

  cycle after the write, if at all. In the Tx example, writing value

  0xacdc filled the FIFO

\item   Setting we=1 when FIFO is full has no effect

\item   Setting re=1 when FIFO is empty has no effect

\item Received data, addr/av and command appear to interface, if FIFO

  was empty before. IP can use them directly. They are ``removed'' only

  when read enable is activated o Checking empty==0 ensures validity

\item Data and command values are undefined when FIFO is empty. Most

  likely the old values remain

\end{enumerate}

A Simple example VHDL code can be found in SVN

/release\_1/lib/hw\_lib/ips/computation/image\_xor/tb/tb\_image\_xor\_linemaker.vhd

It shows how to send address and data.

Fig.~\ref{fig:ip_fsm} shows the simple example FSM of the IP.

\begin{figure*}

  \begin{center}

    {\includegraphics[width=0.9\textwidth]{../Fig/Eps/fig_ip_fsm.eps}}

    \caption{Example FSM of an IP}

    \label{fig:ip_fsm}

  \end{center}

\end{figure*}

Sometimes the output registers of the IP may cause unexpected behavior

for novices. Even if FIFO appears ``not full'', IP cannot necessarily

write new data. That happens if it was already writing and there was

only one place left at the FIFO. Hence, remember to check if IP is

already writing!

The following code snippet should clarify correct writing

\begin{lstlisting}[language=vhdl, style=console, basicstyle=\footnotesize,

    title={Example code of IP's sending control}]

if (we_r ='1' and one_p_in='1') or full_in ='0' then

 we_r   <= '0'; //FIFO is becoming or already full

else

 we_r   <= '1'; // There is room in FIFO

 data_r <= new_value;

end if;

\end{lstlisting}

HIBI wrapper shows the data as soon as it comes from the bus. Same

data might get used (counted) twice, if IP only checks the empty

signal. Remember to check if IP is already reading!  The following

code snippet should clarify correct reading

\begin{lstlisting}[language=vhdl, style=console, basicstyle=\footnotesize,

    title={Example code of IP's reception handling}]