LPTIM-Tick-U5

FreeRTOS Tick/Tickless via LPTIM in STM32U

Use LPTIM for the FreeRTOS tick instead of the SysTick Timer for ultra-low-power applications.

• No drift or slippage in kernel time
• Use STOP modes even while FreeRTOS timers are running or delays are underway
• For any STM32 with LPTIM (see LPTIM-Tick)

This repository demonstrates adaptation of the lptimTick.c gist to the STM32U series MCUs. The specific target platform is the ST B-U585I-IOT02A (STM32U585). The project uses STM32CubeIDE and its integrated code-generation tool (STM32CubeMX). However, lptimTick.c is compatible with any toolchain supported by FreeRTOS.

For a thorough evaluation, this project can be built without tickless idle, with the default tickless idle, or with the custom tickless idle provided by lptimTick.c. See branches for additional evaluation options.

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B-U585I-IOT02A Demo

Press the blue button to cycle between tests:

1. Maintain kernel time only. LED blinks every 5 seconds.
2. Validate tick timing. LED blinks every 2 seconds.
3. Stress test tick timing. LED blinks every second.

Tests 2 and 3 display live test results to a serial terminal. Connect to the STLink Virtual COM Port at 115200 8N1. Additionally, the LED blinks twice (instead of just once) in case of test failure.

Test Results

Current readings shown are averages, not including the LED

With lptimTick.c (configUSE_TICKLESS_IDLE 2)

• Test 1: 9μA, no drift
• Test 2: 35μA, no drift
• Test 3: 57μA, no drift

Default tickless idle (configUSE_TICKLESS_IDLE 1)

• Test 1: 0.49mA, trivial drift
• Test 2: 0.50mA, trivial drift
• Test 3: 0.51mA, trivial drift (with kernel v10.5.1 or newer)

Tickless disabled (configUSE_TICKLESS_IDLE 0)

• Test 1: 1.21mA, no drift
• Test 2: 1.21mA, no drift
• Test 3: 1.21mA, no drift

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Application Requirements

1. You must ensure that no combination of your application's interrupt handlers and your code in configPOST_SLEEP_PROCESSING() can delay the tick ISR more than one tick period. To help your application meet this requirement, consider setting configTICK_INTERRUPT_PRIORITY to configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY, and make sure to implement configPOST_SLEEP_PROCESSING() so it executes quickly. See vUlpPostSleepProcessing() for an example.