5. AXI4 to APB Bridge¶

An AXI4 master device can be configured to work on an APB cluster as a master using the Axi2Apb bridge. This module implements a bridge/adapter which can be used to convert AXI-4 transactions into APB (a.k.a APB4, a.k.a APBv2.0) transactions. This bridge acts as a slave on the AXI4 interface and as a master on an APB interface. Both the protocols are little endian. The bridge is parameterized to handle different address and data sizes of either side with the following contraints:

The AXI4 address size must be greater than or equal to the APB address size.
The AXI4 data size must be greater than of equal to the APB data size.
The AXI4 data and APB data should both be byte-multiples

The bridge also supports spliting of read/write bursts from the AXI4 side to individual requests on the APB cluster.

A Connectable instance is also provided which can directly connect an AXI4 master interface to a APB-slave interface.

The verilog RTL will have a AXI4 slave interface which should be connected to a AXI4 master device and will have an APB master interface on the other side which should be connected to an APB slave cluster.

5.1. Parameters¶

The bridge interface has the following parameters:

Table 5.1 AXI4 to APB Bridge Interface Parameters¶
Parameter Name	Description
`axi_id`	size of the id fields on the AXI4 interface
`axi_addr`	size of the address fields on the read and write channels of the AXI4 interface
`axi_data`	size of the data fields on the read-response and write-data channels of the AXI4 interface
`apb_addr`	size of the address field on the APB interface
`apd_data`	size of the read and write data fields on the APB interface
`user`	size of the user field on both the AXI and the APB sides.

5.2. Micro Architecture¶

Since the APB is a single channel bus and the AXI4 has separate read and write channels, the read requests from the AXI4 are given priority over the write requests occurring in the same cycle. At any point of time only a single requests (burst read or burst write) are served, and the next request is picked up only when the APB has responded to all the bursts from the previous requests.

The transaction ID (awid or arid) received is stored in the conversion bridge, and retrieved during response transfers as BID or RID.

5.2.1. Differing Address sizes¶

When the AXI4 and APB address sizes are different, then the lower bits of the AXI4 addresses are used on the APB side.

5.2.2. Differing Data sizes¶

When the AXI4 and APB data sizes are different, each single beat of the AXI4 request (read or write) is split into multiple smaller child bursts (sent as individual APB requests) which matches APB data size. A beat is complete only when its corresponding child-bursts are over. The next single-beat address is generated based on the burst-mode request and the burst size. Thus, the bridge can support all AXI4 burst-modes: incr, fixed and wrap.

Also when the source (AXI4 side) data size is larger then the target (APB side), the data bytes and the write strobes are aligned based on the address to reflect correctly on the target side.

When instantiated with same data-sizes, the child-burst logic is ommitted.

5.2.3. Error mapping¶

The APB PSLVERR is mapped to the AXI4 SLVERR.

5.3. Using the AXI4 to APB Bridge¶

The IP is designed in BSV and available at: https://gitlab.com/incoresemi/blocks/fabrics The following steps demonstrate on how to configure and generate verilog RTL of the cross-bar IP.

Note

The user is expected to have the downloaded and installed open-source bluespec compiler available at: https://github.com/BSVLang/Main

5.3.1. Configuration and Generation¶

Setup:

The IP uses the python based cogapp tool to generate bsv files with cofigured instances. Steps to install the required tools to generate the configured IP in verilog RTL can be found in Appendix. Python virtual environment needs to be activated before proceeding to the following steps.

Clone the repo:

git clone https://gitlab.com/incoresemi/blocks/fabrics.git
./manager.sh update_deps
cd bridges/test

Configure Design:

The yaml file: axi2apb_bridge_config.yaml is used for configuring the crossbar. Please refer to Table 5.1 for information on the parameters used in the yaml file.
Generate Verilog: use the following command with required settings to generate verilog for synthesis/simulation:
```
make TOP_FILE=axi2apb_bridge.bsv TOP_MODULE=mkaxi2apb_bridge generate_instances
```
The generated verilog file is available in: build/hw/verilog/mkaxi2apb_bridge.v
Interface signals: in the generated verilog, all the AXI4 signals start with the prefix AXI4_ and the APB signals start with the prefix APB_. Since the IP is a synchronous IP, the same clock and reset (active-low) signals (CLK and RST_N) are used by all channles across all devices.
Simulation: The top module for simulation is mkaxi2apb_bridge. Please follow the steps mentioned in Section 8.2 when compiling the top-module for simulation

5.3.2. Verilog Signals¶

Table 5.2 describes the signals in the generated verilog for the following configuration

axi_id: 4
axi_addr:  32
axi_data:  32
apb_addr:  24
apb_data:  16
user    :  0

Table 5.2 AXI4 to APB bridge interface signals in from verilog¶
Signal Names	Direction	Size(Bits)	Description
CLK	Input	1	clock for all channels
RST_N	Input	1	an active low reset
AXI4_AWREADY	Output	1	signal sent to axi4 master
AXI4_WREADY	Output	1	signal sent to axi4 master
AXI4_BVALID	Output	1	signal sent to axi4 master
AXI4_BID	Output	4	signal sent to axi4 master
AXI4_BRESP	Output	2	signal sent to axi4 master
AXI4_ARREADY	Output	1	signal sent to axi4 master
AXI4_RVALID	Output	1	signal sent to axi4 master
AXI4_RID	Output	4	signal sent to axi4 master
AXI4_RDATA	Output	32	signal sent to axi4 master
AXI4_RRESP	Output	2	signal sent to axi4 master
AXI4_RLAST	Output	1	signal sent to axi4 master
APB_PADDR	Output	24	signal sent to apb slaves
APB_PROT	Output	3	signal sent to apb slaves
APB_PENABLE	Output	1	signal sent to apb slaves
APB_PWRITE	Output	1	signal sent to apb slaves
APB_PWDATA	Output	16	signal sent to apb slaves
APB_PSTRB	Output	2	signal sent to apb slaves
APB_PSEL	Output	1	signal sent to apb slaves
AXI4_AWVALID	Input	1	signal driven by axi4 master
AXI4_AWID	Input	4	signal driven by axi4 master
AXI4_AWADDR	Input	32	signal driven by axi4 master
AXI4_AWLEN	Input	8	signal driven by axi4 master
AXI4_AWSIZE	Input	3	signal driven by axi4 master
AXI4_AWBURST	Input	2	signal driven by axi4 master
AXI4_AWLOCK	Input	1	signal driven by axi4 master
AXI4_AWCACHE	Input	4	signal driven by axi4 master
AXI4_AWPROT	Input	3	signal driven by axi4 master
AXI4_AWQOS	Input	4	signal driven by axi4 master
AXI4_AWREGION	Input	4	signal driven by axi4 master
AXI4_WVALID	Input	1	signal driven by axi4 master
AXI4_WDATA	Input	32	signal driven by axi4 master
AXI4_WSTRB	Input	4	signal driven by axi4 master
AXI4_WLAST	Input	1	signal driven by axi4 master
AXI4_BREADY	Input	1	signal driven by axi4 master
AXI4_ARVALID	Input	1	signal driven by axi4 master
AXI4_ARID	Input	4	signal driven by axi4 master
AXI4_ARADDR	Input	32	signal driven by axi4 master
AXI4_ARLEN	Input	8	signal driven by axi4 master
AXI4_ARSIZE	Input	3	signal driven by axi4 master
AXI4_ARBURST	Input	2	signal driven by axi4 master
AXI4_ARLOCK	Input	1	signal driven by axi4 master
AXI4_ARCACHE	Input	4	signal driven by axi4 master
AXI4_ARPROT	Input	3	signal driven by axi4 master
AXI4_ARQOS	Input	4	signal driven by axi4 master
AXI4_ARREGION	Input	4	signal driven by axi4 master
AXI4_RREADY	Input	1	signal driven by axi4 master
APB_PREADY	Input	1	signal driven by the apb slaves
APB_PRDATA	Input	16	signal driven by the apb slaves
APB_PSLVERR	Input	1	signal driven by the apb slaves