Merge branch 'feature/32k_xtal' into 'master'

add support for 32k XTAL as RTC_SLOW_CLK source

- RTC_CNTL_SLOWCLK_FREQ define is removed; rtc_clk_slow_freq_get_hz
  function can be used instead to get an approximate RTC_SLOW_CLK
  frequency

- Clock calibration is performed at startup. The value is saved and used
  for timekeeping and when entering deep sleep.

- When using the 32k XTAL, startup code will wait for the oscillator to
  start up. This can be possibly optimized by starting a separate task
  to wait for oscillator startup, and performing clock switch in that
  task.

- Fix a bug that 32k XTAL would be disabled in rtc_clk_init.

- Fix a rounding error in rtc_clk_cal, which caused systematic frequency
  error.

- Fix an overflow bug which caused rtc_clk_cal to timeout early if the
  slow_clk_cycles argument would exceed certain value

- Improve 32k XTAL oscillator startup time by introducing bootstrapping
  code, which uses internal pullup/pulldown resistors on 32K_N/32K_P
  pins to set better initial conditions for the oscillator.

Ref TW11683.

Ref https://esp32.com/viewtopic.php?f=13&t=1570

Fixes https://github.com/espressif/esp-idf/issues/337.

See merge request !696

components/soc/esp32/include/soc/cpu.h

@@ -73,15 +73,6 @@ static inline void cpu_configure_region_protection()
     cpu_write_itlb(0x20000000, 0);
 }
 
-/**
- * @brief Set CPU frequency to the value defined in menuconfig
- *
- * Called from cpu_start.c, not intended to be called from other places.
- * This is a temporary function which will be replaced once dynamic
- * CPU frequency changing is implemented.
- */
-void esp_set_cpu_freq(void);
-
 /**
  * @brief Stall CPU using RTC controller
  * @param cpu_id ID of the CPU to stall (0 = PRO, 1 = APP)

components/soc/esp32/include/soc/rtc.h

@@ -168,6 +168,15 @@ void rtc_clk_32k_enable(bool en);
  */
 bool rtc_clk_32k_enabled();
 
+/**
+ * @brief Enable 32k oscillator, configuring it for fast startup time.
+ * Note: to achieve higher frequency stability, rtc_clk_32k_enable function
+ * must be called one the 32k XTAL oscillator has started up. This function
+ * will initially disable the 32k XTAL oscillator, so it should not be called
+ * when the system is using 32k XTAL as RTC_SLOW_CLK.
+ */
+void rtc_clk_32k_bootstrap();
+
 /**
  * @brief Enable or disable 8 MHz internal oscillator
  *
@@ -229,6 +238,20 @@ void rtc_clk_slow_freq_set(rtc_slow_freq_t slow_freq);
  */
 rtc_slow_freq_t rtc_clk_slow_freq_get();
 
+/**
+ * @brief Get the approximate frequency of RTC_SLOW_CLK, in Hz
+ *
+ * - if RTC_SLOW_FREQ_RTC is selected, returns ~150000
+ * - if RTC_SLOW_FREQ_32K_XTAL is selected, returns 32768
+ * - if RTC_SLOW_FREQ_8MD256 is selected, returns ~33000
+ *
+ * rtc_clk_cal function can be used to get more precise value by comparing
+ * RTC_SLOW_CLK frequency to the frequency of main XTAL.
+ *
+ * @return RTC_SLOW_CLK frequency, in Hz
+ */
+uint32_t rtc_clk_slow_freq_get_hz();
+
 /**
  * @brief Select source for RTC_FAST_CLK
  * @param fast_freq clock source (one of rtc_fast_freq_t values)

components/soc/esp32/include/soc/rtc_cntl_reg.h

@@ -239,9 +239,6 @@
 #define RTC_CNTL_TIME_VALID_V  0x1
 #define RTC_CNTL_TIME_VALID_S  30
 
-/* frequency of RTC slow clock, Hz */
-#define RTC_CNTL_SLOWCLK_FREQ   150000
-
 #define RTC_CNTL_TIME0_REG          (DR_REG_RTCCNTL_BASE + 0x10)
 /* RTC_CNTL_TIME_LO : RO ;bitpos:[31:0] ;default: 32'h0 ; */
 /*description: RTC timer low 32 bits*/