diff --git a/artiq/firmware/libboard/si5324.rs b/artiq/firmware/libboard/si5324.rs
index ff2e44ffb..8fad61ca7 100644
--- a/artiq/firmware/libboard/si5324.rs
+++ b/artiq/firmware/libboard/si5324.rs
@@ -145,12 +145,12 @@ pub fn setup_hitless_clock_switching(settings: &FrequencySettings) -> Result<()>
         return Err("Si5324 does not have expected product number");
     }
 
-    write(0,   0b01010100)?;        // FREE_RUN=1
-    write(1,   0b11100100)?;        // CK_PRIOR2=1 CK_PRIOR1=0
-    write(2,   0b0010 | (4 << 4))?; // BWSEL=4
-    write(3,   0b0101 | 0x10)?;     // SQ_ICAL=1
-    write(4,   0b10010010)?;        // AUTOSEL_REG=b10
-    write(6,            0x07)?;     // SFOUT1_REG=b111
+    write(0,   read(0)? | 0x40)?;                 // FREE_RUN=1
+    write(1,   (read(1)? & 0xf0) | 0b0100)?;      // CK_PRIOR2=1 CK_PRIOR1=0
+    write(2,   (read(2)? & 0x0f) | (4 << 4))?;    // BWSEL=4
+    write(3,   read(3)? | 0x10)?;                 // SQ_ICAL=1
+    write(4,   (read(4)? & 0x3f) | (0b10 << 6))?; // AUTOSEL_REG=b10
+    write(6,   (read(6)? & 0xc0) | 0b001111)?;    // SFOUT2_REG=b001 SFOUT1_REG=b111
     write(25,  (s.n1_hs  << 5 ) as u8)?;
     write(31,  (s.nc1_ls >> 16) as u8)?;
     write(32,  (s.nc1_ls >> 8 ) as u8)?;
@@ -164,8 +164,8 @@ pub fn setup_hitless_clock_switching(settings: &FrequencySettings) -> Result<()>
     write(46,  (s.n32    >> 16) as u8)?;
     write(47,  (s.n32    >> 8)  as u8)?;
     write(48,  (s.n32)          as u8)?;
-    write(137,          0x01)?;     // FASTLOCK=1
-    write(136,          0x40)?;     // ICAL=1
+    write(137, read(137)? | 0x01)?;               // FASTLOCK=1
+    write(136, read(136)? | 0x40)?;               // ICAL=1
 
     let t = clock::get_ms();
     while !locked()? {