Configurable firmware constants

You can configure your firmware constants, such as a WiFi password, or a LED color with the help of the flashing tool. To do that you'll need to include special configurable constant tags in your source code. Configuration inputs will automatically appear on your project's page after your firmware build is uploaded.

Examples: blink_simple (modified, Pico 2), access_point (modified, Pico w)

In this article we'll explore the ways to use this feature in your C project.

Setup

We need to update the linker scripts included in your copy of pico-sdk. We're doing this so that the configuration constants end up at the beginning of the UF2 firmware blocks.

You can find the linker scripts like this (Linux/WSL):

~/Documents/pico-sdk$ find -name memmap_default.ld
./src/rp2_common/pico_crt0/rp2350/memmap_default.ld
./src/rp2_common/pico_crt0/rp2040/memmap_default.ld

Please open the indicated files, and insert the .configurables section config just below .rodata.

.rodata : {
    (...)
} > FLASH

.configurables : SUBALIGN(256) {
    *(.configurables*)
} > FLASH

Enums

Enum tag struct

typedef struct ConfigurableEnum {
    uint64_t magic1;
    int32_t type;
    int32_t value;
    char name[32];
    char options[128];
    uint64_t magic2;
} ConfigurableEnum;
            

Field description

• magic1: A unique FlashMyPico tag constant (1) (0xD6C9CD9844A1167A).
• type: Always 2 for an enum.
• value: Configured by the flashing tool. May contain a default value.
• name: The name displayed on your project page.
• options: Available configuration options (see the definition below for the format).
• magic2: A unique FlashMyPico tag constant (2) (0xF5A1E6B1EBB053C5).

Enum example definition

Remember to put your constants in their separate sections (alter the section name attribute).

__attribute__((section(".configurables_led_delay_ms")))
const volatile ConfigurableEnum led_delay = {
    0xD6C9CD9844A1167A, // magic1
    2, // type
    250, // value
    "Blinking speed", // name
    "normal:250;fast:100;ultrafast:50;slow:600", // options
    0xF5A1E6B1EBB053C5 // magic2
};
            

Try it out

You can try this out with the blink_simple code example. You'll need to modify the example code to include the above struct declaration, and the constant definition.

Click here for an already modified example, ready to be flashed.

String constants

String tag struct

typedef struct {
    uint64_t magic1;
    int32_t type;
    int32_t padding1;
    char name[32];
    char value[128];
    uint64_t magic2;
} ConfigurableString;
            

Field description

• magic1: A unique FlashMyPico tag constant (1) (0xD6C9CD9844A1167A).
• type: Always 3 for a string.
• padding1: Skip.
• name: The name displayed on your project page.
• value: Configured by the flashing tool. May contain a default value.
• magic2: A unique FlashMyPico tag constant (2) (0xF5A1E6B1EBB053C5).

String example definition

Remember to put your constants in their separate sections (alter the section name attribute).

__attribute__((section(".configurables_ap_name")))
const volatile ConfigurableString cfg_ap_name = {
    0xD6C9CD9844A1167A,
    3,
    0,
    "AP name",
    "picow_test",
    0xF5A1E6B1EBB053C5
};

__attribute__((section(".configurables_ap_password")))
const volatile ConfigurableString cfg_ap_password = {
    0xD6C9CD9844A1167A,
    3,
    0,
    "AP password",
    "password",
    0xF5A1E6B1EBB053C5
};
            

Try it out

You can try this out with the Pico W access_point code example. You'll need to modify the example code to include the above struct declaration, and the constant definition. Please note that all reads from the ConfigurableString instances must be performed through a const volatile char * pointer. Otherwise, your compiler may optimize the read away, leaving the default value in its place.

Click here for an already modified example, ready to be flashed.

Integer constants

Integer tag struct

typedef struct {
    uint64_t magic1;
    int32_t type;
    int32_t value;
    char name[32];
    uint64_t magic2;
} ConfigurableInt32;
            

Field description

• magic1: A unique FlashMyPico tag constant (1) (0xD6C9CD9844A1167A).
• type: Always 1 for an integer.
• value: Configured by the flashing tool. May contain a default value.
• name: The name displayed on your project page.
• magic2: A unique FlashMyPico tag constant (2) (0xF5A1E6B1EBB053C5).

Integer example definition

Remember to put your constants in their separate sections (alter the section name attribute).

__attribute__((section(".configurables_led_delay_ms")))
const volatile ConfigurableInt32 led_delay = {
    0xD6C9CD9844A1167A,
    1,
    250,
    "LED_DELAY_MS",
    0xF5A1E6B1EBB053C5,
};
            

Try it out

You can try this out with the blink_simple code example. You'll need to modify the example code to include the above struct declaration, and the constant definition.

Things to keep in mind

• This feature does not work with signed firmware images. Please let us know if you'd like it changed.
• Similarly, this feature disables the check firmware version feature.