ESP32: dynamic frequency scaling affects RTC accuracy (IDFGH-7771)

[Andste82]

Environment

• Development Kit: none
• Module or chip used: ESP32-WROVER-IE
• IDF version: v4.4.1
• Build System: CMake
• Compiler version: xtensa-esp32-elf-gcc.exe (crosstool-NG esp-2021r2-patch3) 8.4.0
• Operating System: Linux
• Using an IDE?: No
• Power Supply: external 3.3V

Problem Description

If dynamic frequency scaling is active, the RTC runs too fast. An external oscillator is not available, so only the internal ones can be used.

• Internal 150 kHz RC oscillator
• Internal 8.5 MHz oscillator, divided by 256 (~33 kHz)

In both configurations the clock runs too fast.

No sleep functions are used.

Expected Behavior

DFS should not affect RTC.

Actual Behavior

If DFS is active and CPU + APB clock are reduced, the RTC becomes faster. If the APB clock is locked via power-management 'ESP_PM_APB_FREQ_MAX', the accuracy of the RTC remains within limits.

Steps to reproduce

1. activate DFS
2. print time in a cyclic manner

Code snippets

DFS setup:

esp_pm_config_esp32_t pm_config =
{
    .max_freq_mhz = CONFIG_ESP32_DEFAULT_CPU_FREQ_MHZ,
    .min_freq_mhz = (REF_CLK_FREQ * 10) / MHZ,
    .light_sleep_enable = false
};

ESP_ERROR_CHECK(esp_pm_configure(&pm_config));

Lock APB clock:

static esp_pm_lock_handle_t pm_lock_apb;
esp_pm_lock_create(ESP_PM_APB_FREQ_MAX, 0, "lock_apb", &pm_lock_apb);
esp_pm_lock_acquire(pm_lock_apb);

[FanhuaCloud]

I have also encountered the same problem, and currently use an external low-speed 32.768k crystal oscillator to solve the problem