diff --git a/drv/bkp.c b/drv/bkp.c
index 69630b9..798d9e5 100644
--- a/drv/bkp.c
+++ b/drv/bkp.c
@@ -122,19 +122,19 @@ uint32_t compute_prescaler(uint32_t period_ms, enum BkpRtcClockSrc clock_src)
 
 	switch (clock_src) {
 		case BKP_RTC_CLOCK_SRC_LSE:
-			prescaler = 32768000; //32.768kHz
+			prescaler = 32768; //32.768kHz
 			break;
 		case BKP_RTC_CLOCK_SRC_LSI:
-			prescaler = 40000000; //40khz
+			prescaler = 40000; //40khz
 			break;
 		case BKP_RTC_CLOCK_SRC_HSE:
-			prescaler = 62500000; //8Mhz / 128
+			prescaler = 62500; //8Mhz / 128
 			break;
 		default:
 			return 0;
 	}
 
-	return prescaler / period_ms;
+	return (period_ms * prescaler) / 1000;
 }