diff --git a/artiq/runtime/i2c.c b/artiq/runtime/i2c.c
index c8ec3f131..485b9d620 100644
--- a/artiq/runtime/i2c.c
+++ b/artiq/runtime/i2c.c
@@ -139,6 +139,7 @@ int i2c_write(int busno, int b)
     for(i=7;i>=0;i--) {
         /* Set SCL low and set our bit on SDA */
         i2c_scl_o(busno, 0);
+        i2c_halfperiod(); /* make sure SCL has settled low */
         i2c_sda_oe(busno, b & (1 << i) ? 0 : 1);
         i2c_halfperiod();
         /* Set SCL high ; data is shifted on the rising edge of SCL */
@@ -148,8 +149,9 @@ int i2c_write(int busno, int b)
     /* Check ack */
     /* Set SCL low, then release SDA so that the I2C target can respond */
     i2c_scl_o(busno, 0);
-    i2c_halfperiod();
+    i2c_halfperiod(); /* make sure SCL has settled low */
     i2c_sda_oe(busno, 0);
+    i2c_halfperiod();
     /* Set SCL high and check for ack */
     i2c_scl_o(busno, 1);
     i2c_halfperiod();
@@ -182,6 +184,7 @@ int i2c_read(int busno, int ack)
     /* Send ack */
     /* Set SCL low and pull SDA low when acking */
     i2c_scl_o(busno, 0);
+    i2c_halfperiod(); /* make sure SCL has settled low */
     if(ack)
         i2c_sda_oe(busno, 1);
     i2c_halfperiod();