diff --git a/libboard_zynq/src/i2c/mod.rs b/libboard_zynq/src/i2c/mod.rs
index 8290903..04168d4 100644
--- a/libboard_zynq/src/i2c/mod.rs
+++ b/libboard_zynq/src/i2c/mod.rs
@@ -150,16 +150,18 @@ impl I2c {
         self.sda_oe(true);
         self.half_period();
         self.scl_oe(true);
+        while self.scl_i() {}   // ensure SCL is low before start() returns
         // postcondition: SCL and SDA low
         Ok(())
     }
 
     pub fn restart(&mut self) -> Result<(), &'static str> {
-        // precondition SCL and SDA low
+        // precondition: any
         self.sda_oe(false);
         self.half_period();
         self.scl_oe(false);
         self.half_period();
+        // enter the precondition for start(): SCL and SDA high
         self.start()?;
         // postcondition: SCL and SDA low
         Ok(())
@@ -188,6 +190,7 @@ impl I2c {
             self.scl_oe(false);
             self.half_period();
             self.scl_oe(true);
+            while self.scl_i() {}   // ensure SCL is low before start() returns
         }
         self.sda_oe(false);
         self.half_period();
@@ -196,6 +199,7 @@ impl I2c {
         // Read ack/nack
         let ack = !self.sda_i();
         self.scl_oe(true);
+        while self.scl_i() {}   // ensure SCL is low before start() returns
         self.sda_oe(true);
         // postcondition: SCL and SDA low
 
@@ -215,6 +219,7 @@ impl I2c {
             self.half_period();
             if self.sda_i() { data |= 1 << bit }
             self.scl_oe(true);
+            while self.scl_i() {}   // ensure SCL is low before start() returns
         }
         // Send ack/nack
         self.sda_oe(ack);
@@ -222,6 +227,7 @@ impl I2c {
         self.scl_oe(false);
         self.half_period();
         self.scl_oe(true);
+        while self.scl_i() {}   // ensure SCL is low before start() returns
         self.sda_oe(true);
         // postcondition: SCL and SDA low