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Inspect the Written Values of AXI4 Slave Registers by Using the Readback Methods

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This example describes the different techniques to read the AXI4 slave input registers in your design. It shows the process of how to enable readback on AXI4 slave input registers and read the values of AXI4 slave input registers for your design.

Introduction

You can select the AXI4 or AXI4-Lite interface in the IP core generation or the Simulink® Real-Time FPGA I/O workflow in HDL Workflow Advisor. By default, your IP is generated without the readback capability of the AXI4 or AXI4-Lite input registers. When the readback feature is enabled from HDL Workflow Advisor or from command-line interface, you can read the values of input registers. This technique is useful for debugging the input values, which are written into AXI4 or AXI4-Lite interface. The figure shows the AXI4 slave interface and its registers.

Enabling AXI4 Slave Input Register Readback

1. When your design contains the AXI4 or AXI4 Lite interface, you can perform the readback of AXI4 registers. You can use any model which has these interfaces. Use hdlcoder_led_blinking or hdlcoder_led_blinking_4bit model. Open the Simulink model that implements LED blinking by entering this command:

open_system('hdlcoder_led_blinking');

2. Run all the tasks up until Set HDL Options.

3. You can turn on the readback on AXI4 slave input registers by using the HDL workflow Advisor or by using the command-line interface. Generate RTL code and IP Core, by default, the readback on AXI4 slave input register is turned off. To enable this option, select Enable readback on AXI4 slave write registers under the Generate RTL Code and IP Core task of HDL Workflow Advisor.

You can also enable the readback option at the MATLAB command line by using the hdlset_param function.

Read Values of the AXI4 input registers

After setting the readback option for your model, generate the RTL code and the IP Core in the HDL Workflow Advisor or through the command-line interface (CLI). Once the IP core is generated, the IP Core Generation Report is generated in the code generation report. The IP Core Generation Report contains the details about the Target Platform Interface of your model. The figure shows the AXI-Lite interface mapped to the hdlcoder_led_blinking model ports. The table in the IP Core Generation Report shows the interface mapping address of each AXI4 slave register. These addresses are used to read the AXI4 slave input registers.

The AXI4 slave base address is used together with interface mapping address to read the value of AXI4 slave input registers. These address mapping details are used to read the AXI4 slave registers. You can perform the readback of input registers in these ways:

  • Using the MATLAB FPGA Prototyping API Script

  • Using the JTAG AXI Manager

  • Using the Devmem command

To read the value, you can use any of the preceding techniques. Then you must complete all the steps in HDL Workflow Advisor. Once the bitstream is generated and programmed into the target device, perform readback of input registers.

Readback of AXI4 Slave Input Registers by Using MATLAB FPGA Prototyping API Script

You can read AXI4 Slave input registers by using MATLAB® FPGA prototyping API scripts. Once the RTL code and IP core are generated, host interface scripts can be generated by using the Generate Software Interface task from the HDL Workflow Advisor. The option Generate host interface script generates the MATLAB prototyping API scripts.

This option generates two scripts that are interface and setup scripts, shown here for the hdlcoder_led_blinking_4bit model.

The setup function contains commands for the AXI4 slave interfaces that HDL Coder uses to control the DUT ports in the generated HDL IP core, which are mapped to their corresponding interfaces.

The host interface script instantiates this setup function to connect to the target and send read or write commands. You can uncomment and send meaningful data by using the inputs to the DUT in your original model. After interfacing with the hardware, the script disconnects from the hardware resource associated with the FPGA object.

Generate the bitstream and program the target device. To write and read the data for blink_frequency, modify the interface script and run the script. The value 10 is written to the blink_frequency and axi4read shows the readback of blink_frequency.

MATLAB® prototyping API uses JTAG AXI Manager for readback of AXI4 slave input registers. For more information, see Generate and Manage FPGA I/O Host Interface Scripts.

Readback of AXI4 Slave Input Registers by Using JTAG AXI Manager

You can also use separately the JTAG AXI Manager for readback of AXI4 slave input registers. To use JTAG AXI Manager, you must insert the JTAG AXI Manager IP into reference design.

Set the necessary options for enabling readback as mentioned in earlier section and follow the example Use JTAG AXI Manager to Control HDL Coder Generated IP Core for more details.

Readback of AXI4 Slave Input Registers by Using Devmem Command (Probe Registers from Target)

You can use the devmem command in Putty or a hyper terminal. Once you program the bitstream into the target device, open Putty or hyper terminal by using the serial interface. To use devmem command for the hdlcoder_led_blinking model:

1. Read the data from address '400D0100':

   devmem 0x400D0100

You get the value as 0x00000000.

2. Write some value in address '400D0100':

   devmem 0x400D0100 w 0x1

3. Reread the address '400D0100':

  devmem 0x400D0100

You get the value as 0x00000001.

In this way scalars can be read. In the vector data type, you first need to write the data on the register, and then write 0x1 to the strobe address as in vector mode strobe synchronization done for writing. After writing on strobe address, the customer reads the data on the register by using same address. The figure shows the writing and reading of the values in the vector data type by using devmem.

1. Try to read the address 0x400D0100 and 0x400D0104, which shows as 0.

2. Try writing 0x2 on address 0x400D0100, and then write 0x1 on strobe address 0x400D0110.

3. Read the data on address 0x400D0100. It shows as 0x00000002.

4. Write 0x3 on address 0x400D0104, and then write 0x1 on strobe address 0x400D0110.

5. Read the data on address 0x400D0104. It shows 0x00000003.

AXI4 Slave Port to Pipeline Register Ratio

This option is an additional option to optimize your design to meet frequency that you might require. When you have a considerable number of AXI4 slave input ports in the design and AXI4 slave readback of input register is turned on, the design becomes complex and you might not get timing requirements. To optimize the readback capability of the AXI4 slave input registers, you can insert pipelined registers for a number of AXI4 slave ports. You can set the AXI4 slave port to pipeline register ratio option from HDL Workflow Advisor, the CLI, or from the HDL Block properties of the DUT subsystem. The figure shows the HDL Workflow Advisor to set the AXI4 slave port to pipeline register ratio option.

The table shows the drop-down options available in AXI4 slave port to pipeline ratio.

You can also set this option at the MATLAB command line by using the hdlset_param function.

To select this option by using HDL Block properties, right-click DUT subsystem and select HDL Code > HDL Block Properties.

In HDL Block Properties, on the Target Specification tab and set AXI4SlavePortToPipelineRegisterRatio in the IP Core Parameter section.

Once you set the AXI4 slave port to pipeline register ratio by using one of the receding options, follow the steps for the readback as described in the section Read Values of the AXI4 Slave input registers.

Enabling AXI4 Slave Input Register Readback for individual ports

If your design contains a large number of input ports and you enable readback on all of the input ports, your design can consume lot of resources and can have timing violations.

Instead of enabling the global readback on all the input ports, You can enable the readback for individual ports based on your requirement.

1. Below is the example Simulink model which contains large number of input ports.

open_system('hdlcoder_portlevel_readback');

2. Run all the tasks up until Set Target Interface under Set Target of the HDL Workflow Advisor

You can turn on the readback for any of the individual ports by clicking the Options under Interface Options.

In the Set Interface Options window, you can set the Enable write register readback option to on, off or inherit. By default, the Enable write register readback option is set to inherit, where the readback on that port will inherit from the global readback option.

You can also turn on the readback for any individual port at the MATLAB command line by using hdlset_param function in the exported script.

hdlset_param('portlevel_readback/DUT/In1','IOInterfaceOptions',{'RegisterInitialValue','0','EnableReadback','on'});

Utilization Summary for port level readback and global readback

The table below shows the resource utilization for different cases of port level readback and global readback for the above Simulink model portlevel_readback.

The above table shows the utilization summary for three different cases of portlevel and global readback as described below:

Case1: When global readback is off and portlevel readback is also off. In this case, the global and portlevel readback is set to off for all the ports for the above simulink model.

Case2: When global readback is off and portlevel readback is on. In this case, the portlevel readback is enabled for 20 ports for the above simulink model.

Case3: When global readback is on and portlevel readback is off. In this case, the readback is enabled for all the input ports for the above simulink model.

The below chart shows the utilization summary for different cases of port level readback enabled