Implement rtc's LSI calibration

drv/bkp.c

@@ -18,12 +18,14 @@
 
 //--local definitions-----------------------------------------------------------
 
-uint32_t compute_prescaler(uint32_t period_ms, enum BkpRtcClockSrc clock_src);
+static uint32_t compute_prescaler(uint32_t period_ms,
+		enum BkpRtcClockSrc clock_src);
+static void lsi_calib_callback(enum TimIRQSource src);
 
 
 //--local variables-------------------------------------------------------------
 
-//static volatile struct BKP* bkp_regs = (struct BKP*)BKP_BASE_ADDRESS;
+static volatile struct BKP* bkp_regs = (struct BKP*)BKP_BASE_ADDRESS;
 static volatile struct RCC* rcc_regs = (struct RCC*)RCC_BASE_ADDRESS;
 static volatile struct RTC* rtc_regs = (struct RTC*)RTC_BASE_ADDRESS;
 
@@ -103,6 +105,17 @@ uint32_t bkp_read_rtc(void)
 	return time;
 }
 
+uint32_t bkp_read_rtc_div(void)
+{
+	//wait for core registers to be synchronized, immediate most of the time
+	while (rtc_regs->CRL.RSF != 1) {}
+
+	uint32_t div = rtc_regs->DIVH.RTC_DIV << 16;
+	div |= rtc_regs->DIVL.RTC_DIV << 0;
+
+	return div;
+}
+
 void bkp_reset(void)
 {
 	rcc_regs->BDCR.BDRST = 1;
@@ -119,13 +132,44 @@ uint16_t bkp_read_data(enum BkpData data_index)
 	return bkp_regs->DR[data_index].D;
 }
 
+void bkp_calibrate_lsi(enum TimPeriph timer)
+{
+	rcc_enable(RCC_AHB_NONE, RCC_APB1_BKP, RCC_APB2_NONE);
+
+	//do not calibrate if already calibrated
+	if (bkp_read_data(BKP_DATA_LSI_CALIB) != 0) {
+		return;
+	}
+
+	//configure timer to count 1s exactly
+	struct RccClocks clocks;
+	rcc_get_clocks(&clocks);
+
+	tim_set_prescaler(timer, (clocks.tim_freq / 10000)); //10000 to avoid
+														 //prescaler overflow
+	tim_set_auto_reload(timer, 10000);
+	tim_configure_master(timer, TIM_CONFIG_ONE_SHOT
+			| TIM_CONFIG_DIR_UP, TIM_MASTER_CONFIG_MODE_RESET,
+			lsi_calib_callback);
+
+	//configure rtc to tick every 2s so the div value isn't reset during the
+	//timer's delay if the clock is too fast
+	bkp_configure_rtc(2000, BKP_RTC_CLOCK_SRC_LSI, BKP_RTC_IRQ_NONE, nullptr);
+	tim_start(timer);
+
+	while (bkp_read_data(BKP_DATA_LSI_CALIB) == 0) {}
+
+	//TODO reset timer
+}
+
 //--local functions-------------------------------------------------------------
 
 /**
  * Computes the prescaler value based on the clock source and the required
  * period
  */
-uint32_t compute_prescaler(uint32_t period_ms, enum BkpRtcClockSrc clock_src)
+static uint32_t compute_prescaler(uint32_t period_ms,
+		enum BkpRtcClockSrc clock_src)
 {
 	uint32_t prescaler;
 
@@ -134,7 +178,10 @@ uint32_t compute_prescaler(uint32_t period_ms, enum BkpRtcClockSrc clock_src)
 			prescaler = 32768; //32.768kHz
 			break;
 		case BKP_RTC_CLOCK_SRC_LSI:
-			prescaler = 40000; //40khz
+			prescaler = bkp_read_data(BKP_DATA_LSI_CALIB);
+			if (prescaler == 0) {
+				prescaler = 40000; //40khz
+			}
 			break;
 		case BKP_RTC_CLOCK_SRC_HSE:
 			prescaler = 62500; //8Mhz / 128
@@ -146,6 +193,15 @@ uint32_t compute_prescaler(uint32_t period_ms, enum BkpRtcClockSrc clock_src)
 	return (period_ms * prescaler) / 1000;
 }
 
+static void lsi_calib_callback(enum TimIRQSource src)
+{
+	if (src == TIM_IRQ_SOURCE_UPDATE) {
+		//div is decremented from 40kHz * programmed delay (in s)
+		uint32_t lsi_freq = (40000 * 2) - bkp_read_rtc_div();
+		bkp_write_data(BKP_DATA_LSI_CALIB, lsi_freq);
+	}
+}
+
 
 //--ISRs------------------------------------------------------------------------
 

drv/bkp.h

@@ -14,6 +14,7 @@
 //--includes--------------------------------------------------------------------
 
 #include "stdint.h"
+#include "tim.h"
 
 
 //--type definitions------------------------------------------------------------
@@ -98,6 +99,7 @@ void bkp_set_rtc_alam(uint32_t offset);
  * Returns the current counter value of the RTC
  */
 uint32_t bkp_read_rtc(void);
+uint32_t bkp_read_rtc_div(void);
 
 /**
  * Resets the entire backup domain, composed of everything configured through
@@ -108,9 +110,10 @@ void bkp_reset(void);
 void bkp_write_data(enum BkpData data_index, uint16_t data);
 uint16_t bkp_read_data(enum BkpData data_index);
 
+void bkp_calibrate_lsi(enum TimPeriph timer);
+
 //unimplemented functions
 void bkp_configure_tamper();
-void bkp_calibrate_lsi(void);
 void bkp_configure_lse(bool enable);