Attribute Macro ram

#[ram]

Sets which segment of RAM to use for a function or static and how it should be initialized.

Requires the ram feature.

Options

• rtc_fast: Use RTC fast RAM.
• rtc_slow: Use RTC slow RAM. Note: not available on all targets.
• persistent: Persist the contents of the static across resets. See the section below for details.
• zeroed: Initialize the memory of the static to zero. The initializer expression will be discarded. Types used must implement bytemuck::Zeroable.

Using both rtc_fast and rtc_slow or persistent and zeroed together is an error.

persistent

Initialize the memory to zero after the initial boot. Thereafter, initialization is skipped to allow communication across software_reset(), deep sleep, watchdog timeouts, etc.

Types used must implement bytemuck::AnyBitPattern.

Warnings

• A system-level or lesser reset occurring before the ram has been zeroed could skip initialization and start the application with the static filled with random bytes.
• There is no way to keep some kinds of resets from happening while updating a persistent static—not even a critical section.

If these are issues for your application, consider adding a checksum alongside the data.

Examples

#[ram(rtc_fast)]
static mut SOME_INITED_DATA: [u8; 2] = [0xaa, 0xbb];

#[ram(rtc_fast, persistent)]
static mut SOME_PERSISTENT_DATA: [u8; 2] = [0; 2];

#[ram(rtc_fast, zeroed)]
static mut SOME_ZEROED_DATA: [u8; 8] = [0; 8];

See the ram example in the esp-hal repository for a full usage example.