diff --git a/embassy-stm32/src/rcc/f4.rs b/embassy-stm32/src/rcc/f4.rs
index 7aa9f0fd2..2ae0d15cb 100644
--- a/embassy-stm32/src/rcc/f4.rs
+++ b/embassy-stm32/src/rcc/f4.rs
@@ -9,6 +9,7 @@ use crate::gpio::Speed;
 use crate::pac::rcc::vals::{Hpre, Ppre, Sw};
 use crate::pac::{FLASH, PWR, RCC};
 use crate::rcc::{set_freqs, Clocks};
+use crate::rtc::{Rtc, RtcClockSource};
 use crate::time::Hertz;
 use crate::{peripherals, Peripheral};
 
@@ -33,6 +34,7 @@ pub struct Config {
     pub plli2s: Option<Hertz>,
 
     pub pll48: bool,
+    pub rtc: Option<RtcClockSource>,
 }
 
 #[cfg(stm32f410)]
@@ -459,6 +461,18 @@ pub(crate) unsafe fn init(config: Config) {
         })
     });
 
+    match config.rtc {
+        Some(RtcClockSource::LSI) => {
+            RCC.csr().modify(|w| w.set_lsion(true));
+            while !RCC.csr().read().lsirdy() {}
+        }
+        _ => {}
+    }
+
+    config.rtc.map(|clock_source| {
+        Rtc::set_clock_source(clock_source);
+    });
+
     set_freqs(Clocks {
         sys: Hertz(sysclk),
         apb1: Hertz(pclk1),
diff --git a/tests/stm32/src/bin/rtc.rs b/tests/stm32/src/bin/rtc.rs
index 194b153d5..7df415b44 100644
--- a/tests/stm32/src/bin/rtc.rs
+++ b/tests/stm32/src/bin/rtc.rs
@@ -10,13 +10,16 @@ use chrono::{NaiveDate, NaiveDateTime};
 use common::*;
 use defmt::assert;
 use embassy_executor::Spawner;
-use embassy_stm32::pac;
-use embassy_stm32::rtc::{Rtc, RtcConfig};
+use embassy_stm32::rtc::{Rtc, RtcClockSource, RtcConfig};
 use embassy_time::{Duration, Timer};
 
 #[embassy_executor::main]
 async fn main(_spawner: Spawner) {
-    let p = embassy_stm32::init(config());
+    let mut config = config();
+
+    config.rcc.rtc = Some(RtcClockSource::LSI);
+
+    let p = embassy_stm32::init(config);
     info!("Hello World!");
 
     let now = NaiveDate::from_ymd_opt(2020, 5, 15)
@@ -24,13 +27,6 @@ async fn main(_spawner: Spawner) {
         .and_hms_opt(10, 30, 15)
         .unwrap();
 
-    info!("Starting LSI");
-
-    pac::RCC.csr().modify(|w| w.set_lsion(true));
-    while !pac::RCC.csr().read().lsirdy() {}
-
-    info!("Started LSI");
-
     let mut rtc = Rtc::new(p.RTC, RtcConfig::default());
 
     rtc.set_datetime(now.into()).expect("datetime not set");