FPGA Setup

Before following this guide, make sure you’ve followed the dependency installation and software build instructions.

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


To use the OpenTitan 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 on what exactly is needed.

Obtain an FPGA bitstream

To instantiate OpenTitan on an FPGA you will need a bitstream. You can either download an existing bitstream for one of the supported ChipWhisperer boards or you can build it yourself. To ensure the commands shown in this tutorial work for any supported board, we use the BOARD environment variable to specify the target board.

export BOARD=cw340

Download a Pre-built Bitstream

If you are using the ChipWhisperer CW340 board with the Xilinx XCKU095-1FFVA1156C Kintex UltraScale or the CW310 board with the Xilinx Kintex 7 XC7K410T FPGA, you can download the latest passing pre-built bitstream.

For example, to download and unpack the bitstream, run the following:

mkdir -p /tmp/bitstream-latest
pushd /tmp/bitstream-latest
curl https://storage.googleapis.com/opentitan-bitstreams/master/bitstream-latest.tar.gz -o bitstream-latest.tar.gz
tar -xvf bitstream-latest.tar.gz

By default, the bitstream is built with a version of the boot ROM used for testing (called the test ROM; pulled from sw/device/lib/testing/test_rom). There is also a version of the boot ROM used in production (called the ROM; pulled from sw/device/silicon_creator/rom). When the bitstream cache is used in bazel flows, the ROMs from the cache are not used. Instead, the bazel-built ROMs are spliced into the image to create new bitstreams, using the mechanism described in the FPGA Reference Manual. The metadata for the latest bitstream (the approximate creation time and the associated commit hash) is also available as a text file and can be downloaded separately.

Using the @bitstreams repository

OpenTitan’s build system automatically fetches pre-built bitstreams via the @bitstreams repository.

To keep the @bitstreams repository in sync with the current Git revision, install the Git post-checkout hook:

cp util/git/hooks/post-checkout .git/hooks/

Build an FPGA bitstream

Synthesizing a design for an FPGA board is simple with Bazel. While Bazel is the entry point for kicking off the FPGA synthesis, under the hood, it invokes FuseSoC, the hardware package manager / build system supported by OpenTitan. During the build process, the boot ROM is baked into the bitstream. As mentioned above, we maintain two boot ROM programs, one for faster testing (test ROM), and one for production (ROM).

To build an FPGA bitstream with the test ROM for the chosen board, use:

./bazelisk.sh build //hw/bitstream/vivado:fpga_${BOARD}_test_rom

To build an FPGA bitstream with the ROM for the chosen board, use:

./bazelisk.sh build //hw/bitstream/vivado:fpga_${BOARD}_rom_with_fake_keys

Note: Building these bitstreams will require Vivado be installed on your system, with access to the proper licenses, described here. For general software development on the FPGA, Vivado must still be installed, but the Lab Edition is sufficient.

Dealing with FPGA Congestion Issues

The default Vivado tool placement may sometimes result in congested FPGA floorplans. When this happens, the implementation time and results become unpredictable. It may become necessary for the user to manually adjust certain placement. See this comment for a thorough analysis of one such situation and what changes were made to improve congestion.

Opening an existing project with the Vivado GUI

Sometimes, it may be desirable to open the generated project in the Vivado GUI for inspection. To this end, run:

. /tools/Xilinx/Vivado/2021.1/settings64.sh
make -C $(dirname $(find bazel-out/* -wholename '*synth-vivado/Makefile')) build-gui

Now the Vivado GUI opens and loads the project.

Develop with the Vivado GUI

TODO(lowRISC/opentitan#13213): the below does not work with the Bazel FPGA bitstream build flow.

Sometimes it is helpful to use the Vivado GUI to debug a design. FuseSoC (the tool Bazel invokes) 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 not start the synthesis process.

Only create Vivado project directory by using FuseSoC directly (skipping Bazel invocation).

fusesoc --cores-root . run --flag=fileset_top --target=synth --no-export --setup lowrisc:systems:chip_earlgrey_${BOARD}

You can then navigate to the created project directory, and open Vivado

. /tools/Xilinx/Vivado/2021.1/settings64.sh
cd $REPO_TOP/build/lowrisc_systems_chip_earlgrey_${BOARD}_0.1/synth-vivado/vivado

Finally, using the Tcl console, you can kick off the project setup with

source lowrisc_systems_chip_earlgrey_${BOARD}_0.1.tcl

Connecting the ChipWhisperer FPGA board:

CW310 board

The CW310 board supports different power options. It is recommended to power the board via the included DC power adapter. To this end:

  1. Set the SW2 switch (to the right of the barrel connector) up to the 5V Regulator option.
  2. Set the switch below the barrel connector to the right, towards the Barrel option.
  3. Set the Control Power switch (bottom left corner, SW7) to the right.
  4. Ensure the Tgt Power switch (above the fan, S1) is set to the right, towards the Auto option.
  5. Plug the DC power adapter into the barrel jack (J11) in the top left corner of the board.
  6. Use a USB-C cable to connect your PC with the USB-C Data connector (J8) in the lower left corner on the board.

CW340 board

The CW340 board should be powered via the included DC power adapter. To this end:

  1. Set the Control Power switch (top left corner, SW7) to the right.
  2. Ensure the Tgt Power switch (center of the board) is set to the right, towards the Auto option.
  3. Plug the DC power adapter into the barrel jack (J11) in the top left corner of the board.
  4. Use a USB-C cable to connect your PC (host) with the USB-C connector (J28) in the lower left corner on the board.
  5. Set the jumpers JP1 and JP2 to select the UART0 routing:
    1. If set to FTDI the UART0 will (likely) be routed to /dev/ttyUSB2.
    2. If set to SAM the UART0 will be routed to /dev/ttyACM0.

Detecting the board

You can now use the following to monitor output from dmesg:

sudo dmesg -Hw

This should show which serial ports have been assigned, or if the board is having trouble connecting to USB. If dmesg reports a problem you can trigger a USB_RST with SW5. When properly connected, dmesg should identify the board, not show any errors, and the status light should flash (next to SW5). They should be named '/dev/ttyACM*' for CW310 and /dev/ttyACM* + /dev/ttyUSB* + for CW340 depending on the jumpers JP1 and JP2 described above. >e.g. /dev/ttyACM1.

To ensure that you have sufficient access permissions, set up the udev rules as explained in the Vivado installation instructions.

You will then need to run this command to configure the board. You only need to run it once.

./bazelisk.sh run //sw/host/opentitantool -- --interface=${BOARD} fpga set-pll

Check that it’s working by running the demo or a test, such as the uart_smoketest below.

./bazelisk.sh test --test_output=streamed //sw/device/tests:uart_smoketest_fpga_${BOARD}_rom_with_fake_keys


If the tests/demo aren’t working on the FPGA (especially if you get an error like SFDP header contains incorrect signature) then try adding --rcfile= to the set-pll command:

./bazelisk.sh run //sw/host/opentitantool -- --rcfile= --interface=${BOARD} fpga set-pll

It’s also worth pressing the USB_RST and USR_RESET buttons on the FPGA if you face continued errors.

Flash the bitstream onto the FPGA and bootstrap software into flash

There are two ways to load a bitstream on to the FPGA and bootstrap software into the OpenTitan embedded flash:

  1. automatically, on single invocations of ./bazelisk.sh test ....
  2. manually, using multiple invocations of opentitantool, and Which one you use, will depend on how the build target is defined for the software you would like to test on the FPGA. Specifically, for software build targets defined in Bazel BUILD files using the opentitan_test Bazel macro, you will use the latter (automatic) approach. Alternatively, for software build targets defined in Bazel BUILD files using the opentitan_binary Bazel macro, you will use the former (manual) approach.

See below for details on both approaches.

Automatically loading FPGA bitstreams and bootstrapping software with Bazel

A majority of on-device software tests are defined using the custom opentitan_test Bazel macro, which under the hood, instantiates several Bazel native.sh_test rules. In doing so, this macro provides a convenient interface for developers to run software tests on OpenTitan FPGA instances with a single invocation of ./bazelisk.sh test .... For example, to run the UART smoke test (which is an opentitan_test defined in sw/device/tests/BUILD) on FPGA hardware, and see the output in real time, use:

./bazelisk.sh test --test_tag_filters=${BOARD} --test_output=streamed //sw/device/tests:uart_smoketest


./bazelisk.sh test --test_output=streamed //sw/device/tests:uart_smoketest_fpga_${BOARD}_rom_with_fake_keys

Under the hood, Bazel conveniently dispatches opentitantool to both:

  • ensure the correct version of the FPGA bitstream has been loaded onto the FPGA, and
  • bootstrap the desired software test image into the OpenTitan embedded flash.

To get a better understanding of the opentitantool functions Bazel invokes automatically, follow the instructions for manually loading FPGA bitstreams below.

Configuring Bazel to load the Vivado-built bitstream

By default, the above invocations of ./bazelisk.sh test ... use the pre-built (Internet downloaded) FPGA bitstream. To instruct bazel to load the bitstream built earlier, or to have bazel build an FPGA bitstream on the fly, and load that bitstream onto the FPGA, add the --define bitstream=vivado flag to either of the above Bazel commands, for example, run:

./bazelisk.sh test --define bitstream=vivado --test_output=streamed //sw/device/tests:uart_smoketest_fpga_${BOARD}_rom_with_fake_keys

Configuring Bazel to skip loading a bitstream

Alternatively, if you would like to instruct Bazel to skip loading any bitstream at all, and simply use the bitstream that is already loaded, add the --define bitstream=skip flag, for example, run:

./bazelisk.sh test --define bitstream=skip --test_output=streamed //sw/device/tests:uart_smoketest_fpga_${BOARD}_rom_with_fake_keys

Manually loading FPGA bitstreams and bootstrapping OpenTitan software with opentitantool

Some on-device software targets are defined using the custom opentitan_binary Bazel macro. Unlike the opentitan_test macro, the opentitan_binary macro does not instantiate any Bazel test rules under the hood. Therefore, to run such software on OpenTitan FPGA hardware, both a bitstream and the software target must be loaded manually onto the FPGA. Below, we describe how to accomplish this, and in doing so, we shed some light on the tasks that Bazel automates through the use of opentitan_test Bazel rules.

Manually loading a bitstream onto the FPGA with opentitantool

Note: The following examples assume that you have a ~/.config/opentitantool/config with the proper --interface option. For the CW340, its contents would look like:




For CW310.

To flash the bitstream onto the FPGA using opentitantool, use the following command:

If you downloaded the bitstream from the Internet:

./bazelisk.sh run //sw/host/opentitantool -- fpga load-bitstream /tmp/bitstream-latest/lowrisc_systems_chip_earlgrey_${BOARD}_0.1.bit.orig

if you built the bitstream yourself:

./bazelisk.sh run //sw/host/opentitantool -- fpga load-bitstream $(ci/scripts/target-location.sh //hw/bitstream/vivado:fpga_${BOARD}_rom_with_fake_keys)

Depending on the FPGA device, the flashing itself may take several seconds. After completion, a message like this should be visible from the UART:

I00000 test_rom.c:81] Version: earlgrey_silver_release_v5-5886-gde4cb1bb9, Build Date: 2022-06-13 09:17:56
I00001 test_rom.c:87] TestROM:6b2ca9a1
I00002 test_rom.c:118] Test ROM complete, jumping to flash!

Bootstrapping the demo software

The hello_world demo software shows off some capabilities of the OpenTitan hardware. To load hello_world into the FPGA on the ChipWhisperer CW340 board follow the steps shown below.

  1. Generate the bitstream and flash it to the FPGA as described above.

  2. Open a serial console (use the device file determined before) and connect. Settings: 115200 baud, 8 bits per byte, no software flow-control for sending and receiving data.

    screen /dev/ttyACM1 115200,cs8,-ixon,-ixoff
  3. Run opentitantool.

    cd ${REPO_TOP}
    ./bazelisk.sh run //sw/host/opentitantool -- --interface=${BOARD} fpga set-pll # This needs to be done only once.
    ./bazelisk.sh build //sw/device/examples/hello_world:hello_world_fpga_${BOARD}_bin
    ./bazelisk.sh run //sw/host/opentitantool -- bootstrap $(ci/scripts/target-location.sh //sw/device/examples/hello_world:hello_world_fpga_${BOARD}_bin)

    And then output like this should appear from the UART:

    I00000 test_rom.c:81] Version: earlgrey_silver_release_v5-5886-gde4cb1bb9, Build Date: 2022-06-13 09:17:56
    I00001 test_rom.c:87] TestROM:6b2ca9a1
    I00000 test_rom.c:81] Version: earlgrey_silver_release_v5-5886-gde4cb1bb9, Build Date: 2022-06-13 09:17:56
    I00001 test_rom.c:87] TestROM:6b2ca9a1
    I00002 test_rom.c:118] Test ROM complete, jumping to flash!
    I00000 hello_world.c:66] Hello World!
    I00001 hello_world.c:67] Built at: Jun 13 2022, 14:16:59
    I00002 demos.c:18] Watch the LEDs!
    I00003 hello_world.c:74] Try out the switches on the board
    I00004 hello_world.c:75] or type anything into the console window.
    I00005 hello_world.c:76] The LEDs show the ASCII code of the last character.
  4. Observe the output both on the board and the serial console. Type any text into the console window.

  5. Exit screen by pressing CTRL-a k, and confirm with y.

  6. Alternatively you can use the console embedded into opentitantool to see the output from the UART instead of screen.

./bazelisk.sh run //sw/host/opentitantool -- \
--exec "bootstrap $(ci/scripts/target-location.sh //sw/device/examples/hello_world:hello_world_fpga_${BOARD}_bin)"\


If the firmware load fails, try pressing the “USR-RST” button before loading the bitstream.

Debugging with JTAG

Tap Straps

After bootstrapping the firmware, the TAP straps may need to be set. As of this writing, the FPGA images are typically programmed to be in the RMA lifecycle state, and the TAP straps are sampled continuously in that state. To connect the JTAG chain to the CPU’s TAP, adjust the strap values with opentitantool. Assuming opentitantool has been built and that the current directory is the root of the workspace, run these commands:

./bazel-bin/sw/host/opentitantool/opentitantool --interface cw340 \
        --exec "gpio write TAP_STRAP0 false" \
        --exec "gpio write TAP_STRAP1 true" \

CW340 Board

The CW340 supports JTAG-based debugging with OpenOCD and GDB via the FTDI Chip embedded on the board. So no external JTAG adapter is needed.

CW310 Board

The CW310 supports JTAG-based debugging with OpenOCD and GDB via the standard ARM JTAG headers on the board (labeled USR Debug Headers). To use it, program the bitstream and bootstrap the desired firmware, then connect a JTAG adapter to one of the headers. For this guide, the Olimex ARM-USB-TINY-H JTAG adapter was used.

Connect a JTAG adapter to one of the headers. For the Olimex ARM-USB-TINY-H, use the classic ARM JTAG header (J13) and make sure switch S2 is set to 3.3 V. Depending on the adapter’s default state, OpenTitan may be held in reset when the adapter is initially connected. This reset will come under software control once OpenOCD initializes the driver.

Device permissions: udev rules

The JTAG adapter’s device node in /dev must have read-write permissions. Otherwise, OpenOCD will fail because it’s unable to open the USB device. The udev rule below matches the ARM-USB-TINY-H adapter, sets the octal mode mask to 0666, and creates a symlink at /dev/jtag_adapter_arm_usb_tiny_h.

# [/etc/udev/rules.d/90-jtag-adapter.rules]

SUBSYSTEM=="usb", ATTRS{idVendor}=="15ba", ATTRS{idProduct}=="002a", MODE="0666", SYMLINK+="jtag_adapter_arm_usb_tiny_h"

Now, reload the udev rules and reconnect the JTAG adapter.

# Reload the udev rules.
sudo udevadm control --reload-rules
sudo udevadm trigger

# Physically disconnect and reconnect the JTAG adapter, or fake it:
sudo udevadm trigger --verbose --type=subsystems --action=remove --subsystem-match=usb --attr-match="idVendor=15ba"
sudo udevadm trigger --verbose --type=subsystems --action=add --subsystem-match=usb --attr-match="idVendor=15ba"

# Print the permissions of the USB device. This command should print "666".
stat --dereference -c '%a' /dev/jtag_adapter_arm_usb_tiny_h

Connecting OpenOCD

The command below tells OpenOCD to connect to the ChipWhisperer FPGA board via an JTAG adapter. (Note that a different JTAG adapter will require a different config file //third_party/openocd:jtag_*_cfg.)

CW340 - FTDI adapter

./bazelisk.sh run //third_party/openocd -- \
    -f "util/openocd/board/cw340_ftdi.cfg" \
    -c "adapter speed 500; transport select jtag; reset_config trst_only" \
    -f util/openocd/target/lowrisc-earlgrey.cfg

CW310 - Olimex adapter

./bazelisk.sh run //third_party/openocd -- \
    -f "bazel-opentitan/$(./bazelisk.sh outquery //third_party/openocd:jtag_olimex_cfg)" \
    -c "adapter speed 500; transport select jtag; reset_config trst_only" \
    -f util/openocd/target/lowrisc-earlgrey.cfg

Example OpenOCD output:

Open On-Chip Debugger 0.11.0
Licensed under GNU GPL v2
For bug reports, read
trst_only separate trst_push_pull

Info : Hardware thread awareness created
force hard breakpoints
Info : Listening on port 6666 for tcl connections
Info : Listening on port 4444 for telnet connections
Info : clock speed 1000 kHz
Info : JTAG tap: riscv.tap tap/device found: 0x04f5484d (mfg: 0x426 (Google Inc), part: 0x4f54, ver: 0x0)
Info : datacount=2 progbufsize=8
Info : Examined RISC-V core; found 1 harts
Info :  hart 0: XLEN=32, misa=0x40101106
Info : starting gdb server for riscv.tap.0 on 3333
Info : Listening on port 3333 for gdb connections

Note that the reset_config command may need to be adjusted for the particular JTAG adapter in use. TRSTn is available on the 20-pin ARM JTAG header only.

For more guidance on using OpenOCD, see Using 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

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

First, make sure the device software has been built with debug symbols (by default Bazel does not build software with debug symbols). For example, to build and test the UART smoke test with debug symbols, you can add --copt=-g flag to the ./bazelisk.sh test ... command:

./bazelisk.sh test --copt=-g --test_output=streamed //sw/device/tests:uart_smoketest_fpga_cw340_rom_with_fake_keys

Then a connection between OpenOCD and GDB may be established with:

./bazelisk.sh build --config=riscv32 //sw/device/tests:uart_smoketest_prog_fpga_cw340.elf
riscv32-unknown-elf-gdb -ex "target extended-remote :3333" -ex "info reg" \
  "$(./bazelisk.sh outquery --config=riscv32 //sw/device/tests:uart_smoketest_prog_fpga_cw310.elf)"

The above will print out the contents of the registers upon successs. Note that you should have the RISC-V toolchain installed and on your PATH. For example, if you followed the Getting Started instructions, then make sure /tools/riscv/bin is on your PATH.

Common operations with GDB

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

(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

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

Displaying the memory content can also be delegated to OpenOCD

(gdb) monitor mdw 0x200005c0 16

Use monitor help to get a list of supported commands.

To single-step use stepi or step

(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.

(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.

Reproducing FPGA CI Failures Locally

When an FPGA test fails in CI, it can be helpful to run the tests locally with the version of the bitstream generated by the failing CI run. To avoid rebuilding the bitstream, you can download the bitstream artifact from the Azure Pipeline CI run and use opentitantool to load the bitstream manually.

To download the bitstream:

  1. Open your PR on Github and navigate to the “Checks” tab.
  2. On the left sidebar, expand the “Azure Pipelines” menu.
  3. Open the “CI (CW340’s Earl Grey Bitstream)” job and click on “View more details on Azure Pipelines”.
  4. Click on “1 artifact produced”.
  5. Click on the three dots for “partial-build-bin-chip_earlgrey_cw340”.
  6. You can either download the artifact directly or download with the URL.

Note that Azure does not allow you to download the artifact with wget or curl by default, so to use the download URL, you need to specify a user-agent header. For example, to download with curl, you can use the following command

curl --output /tmp/artifact.tar.gz -H 'user-agent: Mozilla/5.0' <download_URL>

After extracting the artifact, the bitstream is located at build-bin/hw/top_earlgrey/lowrisc_systems_chip_earlgrey_cw340_0.1.bit.{splice,orig}. The .splice bitstream has the ROM spliced in, and the .orig bitstream has the test ROM.

Next, load the bitstream with opentitantool, and run the test. The FPGA tests attempt to load the latest bitstream by default, but because we wish to use the bitstream that we just loaded, we need to tell Bazel to skip the automatic bitstream loading.

# Load the bitstream with opentitantool
./bazelisk.sh run //sw/host/opentitantool -- --interface=cw340 fpga load-bitstream <path_to_your_bitstream>

# Run the broken test locally, showing all test output and skipping the bitstream loading
./bazelisk.sh test <broken_test_rule> --define bitstream=skip --test_output=streamed