AHB-Lite Multi-Layer Interconnect Switch

Parameterised AHB-Lite Multi-layer Interconnect Switch

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Datasheet: AHB-Lite Multi-Layer Interconnect Switch

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Contents

Introduction

The Roa Logic AHB-Lite Multi-layer Interconnect Switch, hereafter referred to as ’the Interconnect’, is a fully parameterized high performance, low latency interconnect fabric soft IP for AHB-Lite. It allows a virtually unlimited number of AHB-Lite bus masters and slaves to be connected without the need for bus arbitration to be implemented by the bus masters. Instead, slave side arbitration is implemented for each slave port within the core.

Multi–layer Interconnect Usage Example

The Interconnect supports priority based and round-robin based arbitration when multiple bus masters request access to the same slave port. Typically arbitration completes within 1 clock cycle.

Features

Specifications

Functional Description

The Roa Logic AHB-Lite Multi-layer Interconnect is a highly configurable interconnect fabric for AMBA AHB-Lite based systems, enabling multiple masters to be connected to multiple slaves.

Connections are dynamically created based on which slave a master is addressing, and once created enable direct communication between master and slave without other masters being aware or interfering.

A new connection is typically created within one clock cycle, providing high bandwidth and low latency communication between master and slave.

Example Master / Slave Communication Setup

Master Port

An AHB-Lite bus master connects to a master port of the Multi-layer Interconnect. The master port is implemented as a regular AHB-Lite slave interface thereby allowing support for complex bus structures.

The following figure shows an example bus structure where a bus master – Master-1 – has two directly connected slaves; the Interconnect-Master-Port1 and Slave-4

Connectivity Example for 2 Bus Masters, 4 Slaves

To access a slave, the Interconnect first checks if the designated slave port is available. If it is available the slave port immediately switches to the requesting master. If the slave port is occupied due to another master accessing the slave, the master port generates wait states until the requested slave becomes available. Note the pipelined nature of the AHB-Lite bus may cause a single wait state to be inserted when the slave switches to a new master.

The slave port always retains the connection to the master until another master requests access to that slave port; this enables the original master to request further access to the slave without incurring any delay due to arbitration.

Master Priority

Each master port has a priority level port (mst_priority[ ]).

When multiple masters with different priority levels request access to the same slave port, access is always granted to the master with the highest priority level. If a new master requests access while a transaction is already in progress, access will be granted according to its priority, ahead of any waiting lower priority masters. If masters have the same priority level, then access is granted based on a round-robin scheme.

Master priority may be set dynamically, however assigning a static priority results in a smaller Interconnect and reduces timing paths. The priority value may only be changed while the master port is idle; i.e. mst_HSEL is negated (‘0’) and/or when mst_HTRANS is IDLE.

Bus Locking Support

The priority levels determine the order in which masters are granted access to the slave port. The slave port switches between masters when the current accessing master is idle (mst_HSEL is negated and/or mst_HTRANS = IDLE) or when the current burst completes.

However the current master may lock the bus by asserting HMASTLOCK; this prevents the slave port switching.

Specifying the number of Master Ports

The number of master ports is specified by the MASTERS parameter.

Slave Port

An AHB-Lite bus slave connects to a slave port of the Multi-layer Interconnect. The slave port is implemented as a regular AHB3-Lite master interface thereby allowing support for complex bus structures such as shown below:

Connectivity Example for 2 Bus Masters, 6 Slaves

Address Space Configuration

Each slave port has an address base (slv_addr_base) and address mask (slv_addr_mask) port. Together these set the address range covered by the slave port.

The address base port specifies the base address for the address range covered by the slave port and the address mask port defines the address range covered by the slave port. The internal port select signal is specified as slv_addr_base AND slv_addr_mask.

The address base and address mask values may be changed dynamically, however assigning static values results in a smaller Interconnect and reduces timing paths. Address base and address mask may only be changed when the slave port(s) are idle. Since multiple masters may be active at the same time trying to access the interconnect, special care must be taken to ensure no master accesses the Interconnect while updating the address base and address mask values.

The slave port asserts HSEL when accesses are within the port’s address range. When the port is not being accessed HSEL is negated (‘0’), but HTRANS and other AMBA signals will still provide data. These signals must be ignored while HSEL is negated (‘0’).

The slave port will output the full address, i.e. all HADDR_SIZE bits, on its address bus (slv_HADDR). Connected AMBA slaves should use the relevant least significant bits (LSBs) only.

Example 1
slave_addr_base = 32'h1000_0000
slave_addr_mask = 32'hF000_0000
Address-range = 32'h1000_0000 to 32'h1FFF_FFFF
Example 2
slave_addr_base = 32'h4000_0000
slave_addr_mask = 32'hE000_0000
Address-range = 32'h4000_0000 to 32'h5FFF_FFFF

Slave Port HREADYOUT and HREADY Routing

The slave port has an HREADYOUT port, which is not part of the AHB-Lite specification. It is required to support slaves on the master’s local bus. The HREADY signal, generated by the multiplexor on the master local bus, drives the addressed slave’s HREADYOUT port.

HREADYOUT and HREADY Routing

The simple case of where only one master is connected to a master port or where only a single slave is connected to a slave port is illustrated below.

There are no multiplexors on either the master bus or the slave bus. Since there is no other slave on the master bus, its HREADY signal is only driven by the master port’s HREADYOUT signal. Thus the master port’s HREADYOUT drives both the master’s HREADY input and the master port’s HREADY input.

Similarly since there is no other slave on the slave bus, the slave port’s HREADYOUT signals drives the slave’s HREADY input and the slave’s HREADYOUT signal drives the slave port’s HREADY input.

Single Master/Slave Routing

Specifying the number of Slave Ports

The number of slave ports is specified by the SLAVES parameter.

Configuration

Introduction

The Roa Logic AHB-Lite Multi-layer Interconnect is a highly configurable interconnect fabric for AMBA AHB-Lite based systems. The core parameters and configuration options are described in this section.

Core Parameters

Parameter Type Default Description
HADDR_SIZE Integer 32 Address bus size
HDATA_SIZE Integer 32 Data bus size
MASTERS Integer 3 Number of master ports
SLAVES Integer 8 Number of slave ports
SLAVE_MASK[MASTERS] Array All ’1’s Mask slaves accessible by each master
ERROR_ON_SLAVE_MASK[MASTERS] Array inv(SLAVE_MASK) Enable error reporting for masked slaves
ERROR_ON_NO_SLAVE[MASTERS] Array All ’0’s Disable error reporting when non-mapped address space accessed (to match previous IP releases)
       

Table 1: Core Parameters

HADDR_SIZE

The HADDR_SIZE parameter specifies the width of the address bus for all master and slave ports.

HDATA_SIZE

The HDATA_SIZE parameter specifies the width of the data bus for all master and slave ports.

MASTERS

The MASTERS parameter specifies the number of master ports on the interconnect fabric.

SLAVES

The SLAVES parameter specifies the number of slave ports on the interconnect fabric.

SLAVE_MASK[ ]

The SLAVE_MASK[ ] parameter determines if a master may access a slave. Defining which master may access individual slaves (rather than allowing all masters to access all slaves) may significantly reduce the logic area of the interconnect and improve overall performance.

There is one SLAVE_MASK parameter per master, each SLAVES bits wide. i.e. SLAVE_MASK[ ] is an array of dimensions MASTERS x SLAVES.

Setting a SLAVE_MASK[ ] bit to ’0’ indicates that master cannot access the slave. Conversely, setting a SLAVE_MASK[ ] bit to ’1’ indicates that master may access the slave.

ERROR_ON_SLAVE_MASK[ ]

The ERROR_ON_SLAVE_MASK[ ] parameter enables generating an AHB error response when the master attempts to access a masked slave port.

There is one ERROR_ON_SLAVE_MASK parameter per master, each SLAVES bits wide. i.e. ERROR_ON_SLAVE_MASK[ ] is an array of dimensions MASTERS x SLAVES.

Setting an ERROR_ON_SLAVE_MASK[ ] bit to ’0’ indicates that an AHB error response will not be generated if the master is masked from accessing the corresponding slave. Conversely, setting a ERROR_ON_SLAVE_MASK[ ] bit to ’1’ indicates that an AHB error response will be generated if the master is masked from accessing the corresponding slave.

The default value of ERROR_ON_SLAVE_MASK[ ] is the bitwise inverse of SLAVE_MASK[ ] - i.e. inv(SLAVE_MASK[ ]). If SLAVE_MASK[ ] is assigned a value, then ERROR_ON_SLAVE_MASK[ ] is by default inv(SLAVE_MASK[ ]). The default value of inv(SLAVE_MASK[ ]) causes the core to generate an error response when a masked slave is addressed by a master.

ERROR_ON_NO_SLAVE[ ]

The AHB-Lite Multi-layer Interconnect uses slv_addr_base and slv_addr_mask to decode the target slave port. It can therefore determine if a master attempts to access a non-mapped slave port. This means the switch can generate an AHB error response should a master attempt to access an address which is not mapped to any slave port.

The ERROR_ON_NO_SLAVE[ ] is a is MASTERS bits wide parameter used to enable error response generation for this scenario. Setting a bit of the parameter to ’1’ enables error generation for the corresponding master.

The default value for ERROR_ON_NO_SLAVE[ ] is all bits set to ’0’, disabling this feature to match the behaviour of previous releases of the IP.

Core Macros

Macro Description
RECURSIVE_FUNCTIONS_SUPPORTED Enable use of recursive functions
   

Table 2: Core Macros

RECURSIVE_FUNCTIONS_SUPPORTED

Recursive functions and modules are used within the verilog source code. However EDA tools vary in their support for recursion, with some supporting recursive functions, whereas others support recursive modules or both. For example, Intel Quartus v19.1 and earlier supports recursive modules, but not recursive functions.

To accomodate these toolchain differences, recursive modules are the default method used for implementation. However recursive functions may instead be enabled by setting the synthesis macro RECURSIVE_FUNCTIONS_SUPPORTED

Interfaces

Global Signals

The common signals are shared between all devices on the AHB bus. The AHB-Lite Interconnect has master and slave AHB-Lite buses and they all use the global signals.

Port Size Direction Description
HRESETn 1 Input Asynchronous active low reset
HCLK 1 Input System clock input
       

Table 3: AMBA3 Global Signals

HRESETn

When the active low asynchronous HRESETn input is asserted (‘0’), the core is put into its initial reset state.

HCLK

HCLK is the system clock. All internal logic operates at the rising edge of the system clock. All AHB bus timings are related to the rising edge of HCLK. All master and slave ports must operate at the same HCLK clock.

Master Interface

The master ports are regular AHB3-Lite slave interfaces. All signals are supported. See the AHB-Lite specifications for a complete description of the signals. In addition, a custom master priority port is included per interface to enable prioritisation when multiple masters attempt to simultaneously access the same slave. This prioritisation may be defined statically or dynamically changed during operation.

The AHB-Lite Multi-layer Interconnect implements 1 or more interfaces to AHB-Lite masters as defined by the MASTERS parameter. Therefore the following signals are all arrays reflecting the number of masters supported.

Port Size Direction Description
mst_HSEL 1 Input Bus select
mst_HTRANS 2 Input Transfer type
mst_HADDR HADDR_SIZE Input Address bus
mst_HWDATA HDATA_SIZE Input Write data bus
mst_HRDATA HDATA_SIZE Output Read data bus
mst_HWRITE 1 Input Write select
mst_HSIZE 3 Input Transfer size
mst_HBURST 3 Input Transfer burst size
mst_HPROT 4 Input Transfer protection level
mst_HMASTLOCK 1 Input Transfer master lock
mst_HREADYOUT 1 Output Transfer ready output
mst_HREADY 1 Input Transfer ready input
mst_HRESP 1 Output Transfer response
       

Table 4: Master Interface AHB-Lite Port

Port Size Direction Description
mst_priority clog2(MASTERS) Input master priority Levels
       

Table 5: Master Interface Custom Port

Note: clog2() refers to the System Verilog function by the same name, defined below, and is used to determine the required bitwidth of a bus required to represent the defined range of values:

The system function $clog2 shall return the ceiling of the log base 2 of the argument (the log rounded up to an integer value). The argument can be an integer or an arbitrary sized vector value. The argument shall be treated as an unsigned value, and an argument value of 0 shall produce a result of 0.

mst_HSEL

The master port only responds to other signals on its bus when mst_HSEL is asserted (‘1’). When mst_HSEL is negated (‘0’) the master port considers the bus IDLE and asserts HREADYOUT (‘1’).

mst_HTRANS

mst_HTRANS indicates the type of the current transfer. It is driven to the connected slave.

HTRANS Type Description
00 IDLE No transfer required
01 BUSY Connected master is not ready to accept data, but intents to continue the current burst.
10 NONSEQ First transfer of a burst or a single transfer
11 SEQ Remaining transfers of a burst
     

Table 6: Transfer Type (HTRANS)

mst_HADDR

mst_HADDR is the address bus. Its size is determined by the HADDR_SIZE parameter. It is driven to the connected slave.

mst_HWDATA

mst_HWDATA is the write data bus. Its size is determined by the HDATA_SIZE parameter. It is driven to the connected slave.

mst_HRDATA

mst_HRDATA is the read data bus. Its size is determined by HDATA_SIZE parameter. The connected slave drives it.

mst_HWRITE

mst_HWRITE is the read/write signal. mst_HWRITE asserted (‘1’) indicates a write transfer. It is driven to the connected slave.

mst_HSIZE

mst_HSIZE indicates the size of the current transfer. It is driven to the connected slave.

HSIZE Size Description
000 8bit Byte
001 16bit Half word
010 32bit Word
011 64bits Double word
100 128bit  
101 256bit  
110 512bit  
111 1024bit  
     

Table 7: Transfer Size Values (HSIZE)

mst_HBURST

The burst type indicates if the transfer is a single transfer or part of a burst. It is driven to the connected slave.

HBURST Type Description
000 SINGLE Single access
001 INCR Continuous incremental burst
010 WRAP4 4-beat wrapping burst
011 INCR4 4-beat incrementing burst
100 WRAP8 8-beat wrapping burst
101 INCR8 8-beat incrementing burst
110 WRAP16 16-beat wrapping burst
111 INCR16 16-beat incrementing burst
     

Table 8: Burst Types (HBURST)

mst_HPROT

The protection signals provide information about the bus transfer. They are intended to implement some level of protection. It is driven to the connected slave.

Bit # Value Description
3 1 Cacheable region addressed
  0 Non-cacheable region addressed
2 1 Bufferable
  0 Non-bufferable
1 1 Privileged access
  0 User access
0 1 Data access
  0 Opcode fetch
     

Table 9: Protection Signals (HPROT)

mst_HREADYOUT

When a slave is addressed, the mst_HREADYOUT indicates that the addressed slave finished the current transfer. The Interconnect IP routes the addressed slave’s HREADY signal to the master.

When no slave is addressed, the mst_HREADYOUT signal is generated locally, inside the Interconnect.

mst_HMASTLOCK

The master lock signal indicates if the current transfer is part of a locked sequence, commonly used for Read-Modify-Write cycles. While the mst_HMASTLOCK is asserted, the Interconnect IP cannot switch the addressed slave to another master, even if that master has a higher priority. Instead the current master retains access to slave until it releases mst_HMASTLOCK.

mst_HREADY

mst_HREADY indicates the status of the local HREADY on the master’s local bus. It is routed to the HREADYOUT port of the connected slave.

mst_HRESP

mst_HRESP is the transfer response from the connected slave, it can either be OKAY (‘0’) or ERROR (‘1’). The Interconnect IP routes the connected slave’s HRESP port to mst_HRESP.

mst_priority

mst_priority is a custom port per master interface and defines the priority of the attached master. The width of the bus is calculated as clog2(MASTERS), with the lowest priority value defined as 0 and the highest priority being MASTERS-1.

For example, for a system with 4 masters the width of mst_priority will be 2 bits to enable each master to have a unique prioritisation level, with lowest priority being 0 and highest priority being 3.

Slave Interface

The slave ports are regular AHB-Lite master interfaces.. All signals are supported. In addition each slave port has a non-standard slv_HREADYOUT. See the AHB-Lite specifications for a complete description of the signals.

The AHB-Lite Multi-layer Interconnect implements 1 or more interfaces to AHB-Lite slaves as defined by the SLAVES parameter. Therefore the following signals are all arrays reflecting the number of slaves supported.

Port Size Direction Description
slv_addr_base HADDR_SIZE Input Slave base address
slv_addr_mask HADDR_SIZE Input Slave address space mask
       

Table 10: Slave Interface Customisation Port

Port Size Direction Description
slv_HSEL 1 Output Bus select
slv_HADDR HADDR_SIZE Output Address
slv_HWDATA HDATA_SIZE Output Write data bus
slv_HRDATA HDATA_SIZE Input Read data bus
slv_HWRITE 1 Output Write select
slv_HSIZE 3 Output Transfer size
slv_HBURST 3 Output Transfer burst size
slv_HPROT 4 Output Transfer protection level
slv_HTRANS 2 Input Transfer type
slv_HMASTLOCK 1 Output Transfer master lock
slv_HREADY 1 Input Transfer ready input
slv_HRESP 1 Input Transfer response
       

Table 11: Slave Interface AHB-Lite Port

slv_addr_base

slv_addr_base is a SLAVES sized array of addresses, each HADDR_SIZE bits wide, defining the base address of each attached slave device.

slv_addr_mask

slv_addr_mask is a SLAVES sized array of HADDR_SIZE bit wide signals. Each slv_addr_base address is masked with the corresponding slv_addr_mask value to define the addressable memory space of the attached slave. Setting a bit of slv_addr_mask to ’0’ enables the corresponding address bit.

See section ’Address Space Configuration’ for specific examples.

slv_HSEL

Slaves connected to the slave port must only respond to other signals on the bus when slv_HSEL is asserted (‘1’). When slv_HSEL is negated (‘0’) the interface is idle and the connected slaves must assert their HREADYOUT (‘1’).

slv_HADDR

slv_HADDR is the data address bus. Its size is determined by the HADDR_SIZE parameter. The connected master drives slv_HADDR.

slv_HRDATA

slv_HRDATA is the read data bus. Its size is determined by the HDATA_SIZE parameter. It is driven to the connected master.

slv_HWDATA

slv_HWDATA is the write data bus. Its size is determined by the HDATA_SIZE parameter. The connected master drives slv_HADDR.

slv_HWRITE

slv_HWRITE is the read/write signal. slv_HWRITE asserted (‘1’) indicates a write transfer. The connected master drives slv_HWRITE.

slv_HSIZE

slv_HSIZE indicates the size of the current transfer. The connected master drives slv_HSIZE.

HSIZE Size Description
000 8bit Byte
001 16bit Half word
010 32bit Word
011 64bits Double word
100 128bit  
101 256bit  
110 512bit  
111 1024bit  
     

Table 12: Data Transfer Sizes

slv_HBURST

The burst type indicates if the transfer is a single transfer or part of a burst. The connected master drives it.

HBURST Type Description
000 Single Single access
001 INCR Continuous incremental burst
010 WRAP4 4-beat wrapping burst
011 INCR4 4-beat incrementing burst
100 WRAP8 8-beat wrapping burst
101 INCR8 8-beat incrementing burst
110 WRAP16 16-beat wrapping burst
111 INCR16 16-beat incrementing burst
     

Table 13: Burst Types (HBURST)

slv_HPROT

The data protection signals provide information about the bus transfer. They are intended to implement some level of protection. The connected master drives slv_HPROT.

Bit# Value Description
3 1 Cacheable region addressed
  0 Non-cacheable region addressed
2 1 Bufferable
  0 Non-bufferable
1 1 Privileged access. CPU is not in User Mode
  0 User access. CPU is in User Mode
0 1 Data transfer, always ‘1’
     

Table 14: Data Protection Signals

slv_HTRANS

slv_HTRANS indicates the type of the current data transfer.

slv_HTRANS Type Description
00 IDLE No transfer required
01 BUSY Not used
10 NONSEQ First transfer of an data burst
11 SEQ Remaining transfers of an data burst
     

Table 15: Data Transfer Type

slv_HMASTLOCK

The master lock signal indicates if the current transfer is part of a locked sequence, commonly used for Read-Modify-Write cycles. The connected master drives slv_MASTLOCK.

slv_HREADYOUT

The slv_HREADYOUT signal reflects the state of the connected master port’s HREADY port. It is provided to support local slaves connected directly to the master’s AHB-Lite bus. It is driven by the connected master’s HREADY port.

Note: slv_HREADYOUT is not an AHB-Lite Signal.

slv_HREADY

slv_HREADY indicates whether the addressed slave is ready to transfer data or not. When slv_HREADY is negated (‘0’) the slave is not ready, forcing wait states. When slv_HREADY is asserted (‘0’) the slave is ready and the transfer completed. It is driven to the connected master’s HREADYOUT port.

slv_HRESP

slv_HRESP is the data transfer response, it can either be OKAY (‘0’) or ERROR (‘1’). It is driven to the connected master.

Resources

Below are some example implementations when targeting the Altera Cyclone-V family of FPGAs. All implementations are push button, no effort has been undertaken to reduce area or improve performance.

↓ Res/Config → 10x5 8x5 8x3 5x3 3x5 3x8 5x8 5x10
ALM 6438 4272 2927 1934 1753 2644 4522 5703
Registers 1220 926 842 533 338 377 668 725
Fmax (MHz) 47 61 68 71 109 104 66 63
                 

Table 16: Resource Utilisation Examples

NOTE: Config in the above table refers to the MASTERS x SLAVES configuration of the Interconnect.

Revision History

Date Rev. Comments
13-Oct-2017 1.0 Initial release
16-Sep-2019 1.1 Add SLAVE_MASK[ ] parameter
04-Nov-2020 1.2 Add recursive function support
13-Nov-2020 1.3 Add ERROR_ON_NO_SLAVE[ ] parameter
28-Dec-2023 1.3.1 Correct mst_HRESP[ ] direction definition
     

Table 17: Revision History

  1. The number of bus masters and slaves is physically limited by the timing requirements. ↩