drivers: rtc: STM32U5XX rtc scalers

Allow RTC prescalers to be configurable via dts

Signed-off-by: Jake Greaves <[email protected]>

Author: greavesinator85

drivers/rtc/rtc_ll_stm32.c

@@ -1110,17 +1110,17 @@ BUILD_ASSERT(DT_INST_CLOCKS_HAS_IDX(0, 1), "RTC source clock not defined in the
 static const struct rtc_stm32_config rtc_config = {
 #if DT_INST_CLOCKS_CELL_BY_IDX(0, 1, bus) == STM32_SRC_LSI
 	/* prescaler values for LSI @ 32 KHz */
-	.async_prescaler = 0x7F,
-	.sync_prescaler = 0x00F9,
+	.async_prescaler = DT_INST_PROP_OR(0, async_prescaler, 0x7F),
+	.sync_prescaler = DT_INST_PROP_OR(0, sync_prescaler, 0x00F9),
 #elif DT_INST_CLOCKS_CELL_BY_IDX(0, 1, bus) == STM32_SRC_LSE
 	/* prescaler values for LSE @ 32768 Hz */
-	.async_prescaler = 0x7F,
-	.sync_prescaler = 0x00FF,
+	.async_prescaler = DT_INST_PROP_OR(0, async_prescaler, 0x7F),
+	.sync_prescaler = DT_INST_PROP_OR(0, sync_prescaler, 0x00FF),
 #elif DT_INST_CLOCKS_CELL_BY_IDX(0, 1, bus) == STM32_SRC_HSE
 	/* prescaler values for HSE */
-	.async_prescaler = RTC_HSE_ASYNC_PRESCALER - 1,
-	.sync_prescaler = RTC_HSE_SYNC_PRESCALER - 1,
-	.hse_prescaler = RTC_HSE_PRESCALER,
+	.async_prescaler = DT_INST_PROP_OR(0, async_prescaler, RTC_HSE_ASYNC_PRESCALER - 1),
+	.sync_prescaler = DT_INST_PROP_OR(0, sync_prescaler, RTC_HSE_SYNC_PRESCALER - 1),
+	.hse_prescaler = DT_INST_PROP_OR(0, hse_prescaler, RTC_HSE_PRESCALER),
 #else
 #error Invalid RTC SRC
 #endif

dts/bindings/rtc/st,stm32-rtc.yaml

@@ -14,6 +14,28 @@ properties:
   reg:
     required: true
 
+  sync-prescaler:
+    type: int
+    description: |
+      RTC synchronous prescaler value. Optional, if not set defaulted in code to:
+      - Selected to give 1 Hz tick rate, based on source clock frequency and asynchronous
+        prescaler value.
+
+  async-prescaler:
+    type: int
+    description: |
+      RTC asynchronous prescaler value. Optional, if not set defaulted in code to:
+      - 0x7F if RTC source clock is LSE/LSI
+      - 1 if using counter with CONFIG_COUNTER_RTC_STM32_SUBSECONDS option
+      - 0x7C if RTC source clock is HSE
+
+  hse-prescaler:
+    type: int
+    description: |
+      RTC HSE prescaler value. Applies only when RTC source clock is HSE.
+      Optional, defaulting to the value calculated based on HSE clock frequency
+      to give the highest possibly prescaled source frequency below 1MHz.
+
   calib-out-freq:
     type: int
     description: |