Application Debug


Ledger is currently working on more solutions to improve the coding experience on its products. In particular, a development board and an emulator are in the pipe, but there is not yet an estimated time of arrival for these.


The BOLOS development environment is required for the following article. It applies only for the Nano S, with its latest firmware firmware (1.5.5).

It is possible to install a debugging firmware on the device’s MCU that will enable printing text outputs from the device to a terminal. To do so, follow these steps:

1. First, download the updater and the debug firmware .

2. Exit any instance of Ledger Live, Ledger Chrome App, or any other program able to communicate with a Ledger device.

3. Now, plug your Nano S to your computer while keeping the left button pressed. Keep it pressed until the screen displays BOOTLOADER.

4. Fire a terminal and move to the directory containing the files downloaded at step 1.

  1. Install the updater:

    python -m ledgerblue.loadMCU --targetId 0x01000001 --fileName blup_0.9_misc_m1.hex --nocrc

Wait until BOOTLOADER is displayed again on the device’s screen.

  1. Install the debug firmware:

    python -m ledgerblue.loadMCU --targetId 0x01000001 --fileName mcu_1.7-printf_over_0.9.hex --reverse --nocrc

If you can notice a small dbg block at the bottom of the screen, then it’s a success !


A Nano S with special debug firmware

Uninstalling this special firmware is also very easy, first you need to download the normal firmware, then you can repeat the installation steps 2 to 5.

Finally, flash the normal firmware with this command:

python -m ledgerblue.loadMCU --targetId 0x01000001 --fileName mcu_1.7_over_0.9.hex --reverse --nocrc

The dbg block should now be gone.

PRINTF macro

The debug firmware enables the PRINTF macro, however you have to define it in your app’s Makefile. To do so, add this line in your Makefile: DEFINES += HAVE_SPRINTF HAVE_PRINTF PRINTF=screen_printf

Usually, PRINTF is already defined to void with this line DEFINES   += PRINTF\(...\)=. Check if PRINTF is already defined somewhere else in your Makefile, and comment out this definition so it doesn’t override the one that we just set.


The PRINTF macro is a debugging feature, and as such it is not intended for use in production. When compiling an application for release purpose, please verify that PRINTF is disabled in your Makefile. In other words, in case of release compilation, put back the line DEFINES   += PRINTF\(...\)= and comment out the other one.


The PRINTF macro can only be used in between successive calls to io_exchange. Calling it outside of it will result in unexpected behavior. Behind the scene, PRINTF sends a status on the SEPH. Only one status can be sent in a row, otherwise the SEPH crashes. For this reason, don’t use PRINTF just after status-sending calls, such as UX_DISPLAY. This macro packs a call to io_seproxyhal_display and is often the reason for application crashes. Then is no other work around than to move your call to PRINTF somewhere else in your code.

This macro can be used in your code like a classical printf function from stdio.h. However, it is improved with a neat feature to easily print byte arrays:

uint8_t buffer[4] = {0xDE, 0xAD, 0xBE, 0xEF};

// PRINTF(string, array length, array);
// .*H for uppercase, .*h for lowercase
PRINTF("What a lovely buffer:\n %.*H \n\n", 4, buffer);
PRINTF("I prefer it lower-cased:\n %.*h \n", 4, buffer);

Result of the example code printed inside a terminal

Console Printing

The PRINTF macro triggers messages from the MCU to the host computer through the USB link. We use USBTool to read these messages and print their payload in a terminal.

Unzip the file and execute this command: ./usbtool -v 0x2c97 log

Now you can launch your app on your Nano S, and every PRINTF will end up printed on the host computer, allowing you to debug your program more easily.

PIN bypass

In Ledger app development, it is necessary to enter your PIN code each time you install an unsigned app. In order to do testing during development, this means developers wind up using many painful button presses entering a PIN code compared to relatively few testing their own application code. The Ledger OS (BOLOS) supports installing a custom developer certificate. By installing a custom certificate once on your device you can avoid having to retype your PIN each time you adjust your app. Here are the steps for the Ledger Nano S:

  1. Generate a public / private keypair using the following command:

    foo@bar:~$ python -m ledgerblue.genCAPair
    Public key : 0495331cb86e961fc9cb5792a97737e4204b58be99321dcec463cec3057b3304e9875614004e6e540ab0610a1339fae22df6f6f3ec594912b409d69b72f0eaf390
    Private key: 6c1f1df852255e113b23c2e977d6b5c3ea2aaf411f05a5fdab87a3e8f44468ee

2. Enter recovery mode on your Ledger Nano S. Do this by unplugging it then holding down the right button (near the hinge, away from USB port) while plugging it in again. recovery mode shoudl then appear on the screen. Enter your pin and continue.

3. Load your public key onto the Ledger Nano S. Paste the public key generated at step 1 after --public. You may need to adjust the --targetId constant to match your device. Find the right targetId for your device here. Notice that we must include a --name parameter containing the name of the custom certificate (any string will do):

python -m ledgerblue.setupCustomCA --targetId 0x31100004 --public yourPublicKey --name dev

If you receive the error Invalid status 6985 then please review this link and then go back to step 2. The above command is the simplest that can work however some developers may wish to use the optional --rootPrivateKey option to specify a secure channel encryption key (which is the private key generated at step 1) or use the --name option for convenient labeling according to local convention.

4. Once you have loaded your custom certificate, you can try to load an app you compiled yourself onto your Ledger to see if you are able to bypass the PIN. Before you try it, set the SCP_PRIVKEY environment variable to contain the private key generated at step 1 in your shell or .bashrc:

export SCP_PRIVKEY=yourPrivateKey

and then rebuild/load your app.

For more information see loadpp-py


A side effect of installing a custom CA on your device is that it will from now on fail to pass the Ledger Genuine Check, which is required to install applications from the Ledger Live. To make it genuine again, you should uninstall your custom CA and all the applications installed through it.

Uninstalling a custom CA is very quick:

1. Enter recovery mode on your Ledger Nano S. Do this by unplugging it then holding down the right button (near the hinge, away from USB port) while plugging it in again. recovery mode shouldl then appear on the screen. Enter your pin and continue.

  1. Type this command in your terminal:

    foo@bar:~$ python -m ledgerblue.resetCustomCA --targetId 0x31100004

Find the right targetId for your device here.