---
title: "Getting started on FPGAs"
---

Do you want to try out the lowRISC chip designs, but don't have a couple thousand or million dollars ready for an ASIC tapeout?
Running lowRISC designs on an FPGA board can be the answer!

<!-- TODO: Switch all calls to fusesoc and the Verilated system to refer to Meson, once it supports fusesoc. -->

## Prerequisites

To use the lowRISC Comportable designs on an FPGA you need two things:

* A supported FPGA board
* A tool from the FPGA vendor

Depending on the design/target combination that you want to synthesize you will need different tools and boards.
Refer to the design documentation for information what exactly is needed.

* [Obtain an FPGA board]({{< relref "fpga_boards.md" >}})

Follow the install instructions to [prepare the system]({{< relref "install_instructions#system-preparation" >}}) and to install the [software development tools]({{< relref "install_instructions#software-development" >}}) and [Xilinx Vivado]({{< relref "install_instructions#xilinx-vivado" >}}).

## Create an FPGA bitstream

Synthesizing a design for a FPGA board is done with the following commands.

The FPGA build will pull in a program to act as the boot ROM.
This must be built before running the FPGA build.
This is pulled in from the `sw/device/boot_rom` directory (see the `parameters:` section of the `hw/top_earlgrey/top_earlgrey_nexysvideo.core` file).

To build it:
```console
$ cd $REPO_TOP
$ ./meson_init.sh
$ ninja -C build-out sw/device/boot_rom/boot_rom_export_fpga_nexysvideo
```

In the following example we synthesize the Earl Grey design for the Nexys Video board using Xilinx Vivado 2020.1.

```console
$ . /tools/xilinx/Vivado/2020.1/settings64.sh
$ cd $REPO_TOP
$ ./meson_init.sh
$ ninja -C build-out sw/device/boot_rom/boot_rom_export_fpga_nexysvideo
$ fusesoc --cores-root . run --flag=fileset_top --target=synth lowrisc:systems:top_earlgrey_nexysvideo
```
The fsel_top flag used above is specific to the OpenTitan project to select the correct fileset.

The resulting bitstream is located at `build/lowrisc_systems_top_earlgrey_nexysvideo_0.1/synth-vivado/lowrisc_systems_top_earlgrey_nexysvideo_0.1.bit`.
See the [reference manual]({{< relref "ref_manual_fpga.md" >}}) for more information.

## Connecting the board

* Use a Micro USB cable to connect the PC with the *PROG*-labeled connector on the board.
* Use a second Micro USB cable to connect the PC with the *UART*-labled connector on the board.
* After connecting the UART, use `dmesg` to determine which serial port was assigned. It should be named `/dev/ttyUSB*`, e.g. `/dev/ttyUSB0`.
* Ensure that you have sufficient access permissions to the device, check `ls -l /dev/ttyUSB*`. The udev rules given in the Vivado installation instructions ensure this.

## Flash the bitstream onto the FPGA

To flash the bitstream onto the FPGA you need to use either the Vivado GUI or the command line.

### Using the command line

Use the following command to program the FPGA with fusesoc.

```console
$ . /tools/xilinx/Vivado/2020.1/settings64.sh
$ cd $REPO_TOP
$ fusesoc --cores-root . pgm lowrisc:systems:top_earlgrey_nexysvideo:0.1
```

This should produce a message like this from the UART:

```
Version:    opentitan-snapshot-20191101-1-366-gca61d28
Build Date: 2019-12-13, 13:15:48
Bootstrap requested, initialising HW...
HW initialisation completed, waiting for SPI input...
```

Note: `fusesoc pgm` is broken for edalize versions up to (and including) v0.1.3.
You can check the version you're using with `pip3 show edalize`.
If you have having trouble with programming using the command line, try the GUI.

### Using the Vivado GUI

```console
$ . /tools/xilinx/Vivado/2020.1/settings64.sh
$ cd $REPO_TOP
$ make -C build/lowrisc_systems_top_earlgrey_nexysvideo_0.1/synth-vivado build-gui
```

Now the Vivado GUI opens and loads the project.

* Connect the FPGA board to the PC and turn it on.
* In the navigation on the left, click on *PROGRAM AND DEBUG* > *Open Hardware Manager* > *Open Target* > *Auto Connect*.
* Vivado now enumerates all boards and connects to it.
* Click on *Program Device* in the menu on the left (or at the top of the screen).
* A dialog titled *Program Device* pops up. Select the file `lowrisc_systems_top_earlgrey_nexysvideo_0.1.bit` as *Bitstream file*, and leave the *Debug probes file* empty.
* Click on *Program* to flash the FPGA with the bitstream.
* The FPGA is ready as soon as the programming finishes.


## Testing the demo design

The `hello_world` demo software shows off some capabilities of the design.
In order to load `hello_world` into the FPGA, both the binary and the [loading tool]({{< relref "/sw/host/spiflash/README.md" >}}) must be compiled.
Please follow the steps shown below.

* Generate the bitstream and flash it to the FPGA as described above.
* Open a serial console (use the device file determined before) and connect.
  Settings: 115200 baud, 8N1, no hardware or software flow control.
  ```console
  $ screen /dev/ttyUSB0 115200
  ```
  Note that the Nexsys Video demo program that comes installed on the board runs the UART at 115200 baud as well;
  expect to see different output if that is running.
  This can happen if you connect the serial console before using Vivado to program your new bitstream or you press the *PROG* button that causes the FPGA to reprogram from the code in the on-board SPI flash.
* On the Nexys Video board, press the red button labeled *CPU_RESET*.
* You should see the ROM code report its commit ID and build date.
* Run the loading tool.
  ```console
  $ cd ${REPO_TOP}
  $ ./meson_init.sh
  $ ninja -C build-out sw/device/examples/hello_world/hello_world_export_fpga_nexysvideo
  $ ninja -C build-out sw/host/spiflash/spiflash_export
  $ build-bin/sw/host/spiflash/spiflash --input build-bin/sw/device/examples/hello_world/hello_world_fpga_nexysvideo.bin
  ```

  which should report how the binary is split into frames:

  ```
   Running SPI flash update.
   Image divided into 6 frames.
   frame: 0x00000000 to offset: 0x00000000
   frame: 0x00000001 to offset: 0x000003d8
   frame: 0x00000002 to offset: 0x000007b0
   frame: 0x00000003 to offset: 0x00000b88
   frame: 0x00000004 to offset: 0x00000f60
   frame: 0x80000005 to offset: 0x00001338
   ```

  and then output like this should appear from the UART:
  ```
  Processing frame no: 00000000 exp no: 00000000
  Processing frame no: 00000001 exp no: 00000001
  Processing frame no: 00000002 exp no: 00000002
  Processing frame no: 00000003 exp no: 00000003
  Processing frame no: 00000004 exp no: 00000004
  Processing frame no: 80000005 exp no: 00000005
  bootstrap: DONE!
  INFO: Boot ROM initialisation has completed, jump into flash!
  Hello World! Dec 13 2019 15:06:29
  Watch the LEDs!
  Try out the switches on the board
  or type anything into the console window.
  The LEDs show the ASCII code of the last character.
  GPIO: Switch 7 changed to 1
  FTDI control changed. Enable JTAG
  ```

* Observe the output both on the board and the serial console. Type any text into the console window.
* Exit `screen` by pressing CTRL-a k, and confirm with y.

## Develop with the Vivado GUI

Sometimes it is helpful to use the Vivado GUI to debug a design.
fusesoc makes that easy, with one small caveat: by default fusesoc copies all source files into a staging directory before the synthesis process starts.
This behavior is helpful to create reproducible builds and avoids Vivado modifying checked-in source files.
But during debugging this behavior is not helpful.
The `--no-export` option of fusesoc disables copying the source files into the staging area, and `--setup` instructs fusesoc to only create a project file, but not to run the synthesis process.

```console
$ # only create Vivado project file
$ fusesoc --cores-root . build --no-export --setup lowrisc:systems:top_earlgrey_nexysvideo
```

## Connect with OpenOCD and debug

To connect the FPGA with OpenOCD, run the following command

```console
$ cd $REPO_TOP
$ /tools/openocd/bin/openocd -s util/openocd -f board/lowrisc-earlgrey-nexysvideo.cfg
```

Note that you must use the [RISC-V fork of OpenOCD](https://github.com/riscv/riscv-openocd).
See the [install instructions]({{< relref "install_instructions#openocd" >}}) for guidance on building OpenOCD.

To actually debug through OpenOCD, it must either be connected through telnet or GDB.

### Debug with OpenOCD

The following is an example for using telnet

```console
$ telnet localhost 4444 // or whatever port that is specificed by the openocd command above
$ mdw 0x8000 0x10 // read 16 bytes at address 0x8000
```

### Debug with GDB

An example connection with GDB, which prints the registers after the connection to OpenOCD is established

```console
$ cd $REPO_TOP
$ /tools/riscv/bin/riscv32-unknown-elf-gdb -ex "target extended-remote :3333" -ex "info reg" sw/device/boot_rom/rom.elf
```

#### Common operations with GDB

Examine 16 memory words in the hex format starting at 0x200005c0

```console
(gdb) x/16xw 0x200005c0
```

Press enter again to print the next 16 words.
Use `help x` to get a description of the command.

If the memory content contains program text it can be disassembled

```console
(gdb) disassemble 0x200005c0,0x200005c0+16*4
```

Displaying the memory content can also be delegated to OpenOCD

```console
(gdb) monitor mdw 0x200005c0 16
```

Use `monitor help` to get a list of supported commands.

To single-step use `stepi` or `step`

```console
(gdb) stepi
```

`stepi` single-steps an instruction, `step` single-steps a line of source code.
When testing debugging against the hello_world binary it is likely you will break into a delay loop.
Here the `step` command will seem to hang as it will attempt to step over the whole delay loop with a sequence of single-step instructions which may take quite some time!

To change the program which is debugged the `file` command can be used.
This will update the symbols which are used to get information about the program.
It is especially useful in the context of our `rom.elf`, which resides in the ROM region, which will eventually jump to a different executable as part of the flash region.

```console
(gdb) file sw/device/examples/hello_world/sw.elf
(gdb) disassemble 0x200005c0,0x200005c0+16*4
```

The output of the disassemble should now contain additional information.