diff --git a/artiq/runtime.rs/libksupport/api.rs b/artiq/runtime.rs/libksupport/api.rs
index fd069d8c4..5a12ea691 100644
--- a/artiq/runtime.rs/libksupport/api.rs
+++ b/artiq/runtime.rs/libksupport/api.rs
@@ -105,9 +105,9 @@ static mut API: &'static [(&'static str, *const ())] = &[
     api!(rtio_input_timestamp = ::rtio::input_timestamp),
     api!(rtio_input_data = ::rtio::input_data),
 
-    api!(i2c_init),
-    api!(i2c_start),
-    api!(i2c_stop),
-    api!(i2c_write),
-    api!(i2c_read),
+    api!(i2c_init = ::i2c::init),
+    api!(i2c_start = ::i2c::start),
+    api!(i2c_stop = ::i2c::stop),
+    api!(i2c_write = ::i2c::write),
+    api!(i2c_read = ::i2c::read),
 ];
diff --git a/artiq/runtime.rs/libksupport/i2c.rs b/artiq/runtime.rs/libksupport/i2c.rs
new file mode 100644
index 000000000..8a9f6da2d
--- /dev/null
+++ b/artiq/runtime.rs/libksupport/i2c.rs
@@ -0,0 +1,166 @@
+use board::csr;
+
+fn half_period() {
+    unsafe {
+        csr::timer_kernel::en_write(0);
+        csr::timer_kernel::load_write(csr::CONFIG_CLOCK_FREQUENCY/10000);
+        csr::timer_kernel::reload_write(0);
+        csr::timer_kernel::en_write(1);
+
+        csr::timer_kernel::update_value_write(1);
+        while csr::timer_kernel::value_read() != 0 {
+            csr::timer_kernel::update_value_write(1)
+        }
+    }
+}
+
+#[cfg(has_i2c)]
+mod imp {
+    use board::csr;
+
+    fn sda_bit(busno: u32) -> u32 { 1 << (2 * busno + 1) }
+    fn scl_bit(busno: u32) -> u32 { 1 << (2 * busno) }
+
+    pub fn sda_i(busno: u32) -> bool {
+        unsafe {
+            if busno >= csr::CONFIG_I2C_BUS_COUNT {
+                true
+            } else {
+                csr::i2c::in_read() & sda_bit(busno) != 0
+            }
+        }
+    }
+
+    pub fn sda_oe(busno: u32, oe: bool) {
+        unsafe {
+            let reg = csr::i2c::oe_read();
+            let reg = if oe { reg | sda_bit(busno) } else { reg & !sda_bit(busno) };
+            csr::i2c::oe_write(reg);
+        }
+    }
+
+    pub fn sda_o(busno: u32, o: bool) {
+        unsafe {
+            let reg = csr::i2c::out_read();
+            let reg = if o  { reg | sda_bit(busno) } else { reg & !sda_bit(busno) };
+            csr::i2c::out_write(reg)
+        }
+    }
+
+    pub fn scl_oe(busno: u32, oe: bool) {
+        unsafe {
+            let reg = csr::i2c::oe_read();
+            let reg = if oe { reg | scl_bit(busno) } else { reg & !scl_bit(busno) };
+            csr::i2c::oe_write(reg)
+        }
+    }
+
+    pub fn scl_o(busno: u32, o: bool) {
+        unsafe {
+            let reg = csr::i2c::out_read();
+            let reg = if o  { reg | scl_bit(busno) } else { reg & !scl_bit(busno) };
+            csr::i2c::out_write(reg)
+        }
+    }
+}
+
+// #[cfg(not(has_i2c))]
+// mod imp {
+//     pub fn sda_i(busno: u32) -> bool { true }
+//     pub fn sda_oe(busno: u32, oe: bool) {}
+//     pub fn sda_o(busno: u32, o: bool) {}
+//     pub fn scl_oe(busno: u32, oe: bool) {}
+//     pub fn scl_o(busno: u32, o: bool) {}
+// }
+
+use self::imp::*;
+
+pub extern fn init(busno: u32) {
+    // Set SCL as output, and high level
+    scl_o(busno, true);
+    scl_oe(busno, true);
+    // Prepare a zero level on SDA so that sda_oe pulls it down
+    sda_o(busno, false);
+    // Release SDA
+    sda_oe(busno, false);
+
+    // Check the I2C bus is ready
+    half_period();
+    half_period();
+    if !sda_i(busno) {
+        artiq_raise!("I2CError", "SDA is stuck low")
+    }
+}
+
+pub extern fn start(busno: u32) {
+    // Set SCL high then SDA low
+    scl_o(busno, true);
+    half_period();
+    sda_oe(busno, true);
+    half_period();
+}
+
+pub extern fn stop(busno: u32) {
+    // First, make sure SCL is low, so that the target releases the SDA line
+    scl_o(busno, false);
+    half_period();
+    // Set SCL high then SDA high
+    sda_oe(busno, true);
+    scl_o(busno, true);
+    half_period();
+    sda_oe(busno, false);
+    half_period();
+}
+
+pub extern fn write(busno: u32, data: u8) -> bool {
+    // MSB first
+    for bit in (0..8).rev() {
+        // Set SCL low and set our bit on SDA
+        scl_o(busno, false);
+        sda_oe(busno, data & (1 << bit) == 0);
+        half_period();
+        // Set SCL high ; data is shifted on the rising edge of SCL
+        scl_o(busno, true);
+        half_period();
+    }
+    // Check ack
+    // Set SCL low, then release SDA so that the I2C target can respond
+    scl_o(busno, false);
+    half_period();
+    sda_oe(busno, false);
+    // Set SCL high and check for ack
+    scl_o(busno, true);
+    half_period();
+    // returns true if acked (I2C target pulled SDA low)
+    !sda_i(busno)
+}
+
+pub extern fn read(busno: u32, ack: bool) -> u8 {
+    // Set SCL low first, otherwise setting SDA as input may cause a transition
+    // on SDA with SCL high which will be interpreted as START/STOP condition.
+    scl_o(busno, false);
+    half_period(); // make sure SCL has settled low
+    sda_oe(busno, false);
+
+    let mut data: u8 = 0;
+
+    // MSB first
+    for bit in (0..8).rev() {
+        scl_o(busno, false);
+        half_period();
+        // Set SCL high and shift data
+        scl_o(busno, true);
+        half_period();
+        if sda_i(busno) { data |= 1 << bit }
+    }
+    // Send ack
+    // Set SCL low and pull SDA low when acking
+    scl_o(busno, false);
+    if ack { sda_oe(busno, true) }
+    half_period();
+    // then set SCL high
+    scl_o(busno, true);
+    half_period();
+
+    data
+}
diff --git a/artiq/runtime.rs/libksupport/lib.rs b/artiq/runtime.rs/libksupport/lib.rs
index f46d5efaf..6b285f889 100644
--- a/artiq/runtime.rs/libksupport/lib.rs
+++ b/artiq/runtime.rs/libksupport/lib.rs
@@ -50,6 +50,7 @@ macro_rules! artiq_raise {
 }
 
 mod rtio;
+mod i2c;
 
 use core::{mem, ptr, slice, str};
 use std::io::Cursor;
diff --git a/artiq/runtime/Makefile b/artiq/runtime/Makefile
index 36685661c..a8387466b 100644
--- a/artiq/runtime/Makefile
+++ b/artiq/runtime/Makefile
@@ -4,7 +4,7 @@ include $(MISOC_DIRECTORY)/software/common.mak
 PYTHON ?= python3.5
 
 OBJECTS := flash_storage.o main.o
-OBJECTS_KSUPPORT := ksupport_glue.o artiq_personality.o i2c.o
+OBJECTS_KSUPPORT := ksupport_glue.o artiq_personality.o
 
 RUSTOUT_DIRECTORY := cargo/or1k-unknown-none/debug
 CORE_IO_COMMIT := d40c593f42fafbac1ff3d827f6df96338b5b7d8b
diff --git a/artiq/runtime/i2c.c b/artiq/runtime/i2c.c
deleted file mode 100644
index c8ec3f131..000000000
--- a/artiq/runtime/i2c.c
+++ /dev/null
@@ -1,193 +0,0 @@
-#include <generated/csr.h>
-
-#include "artiq_personality.h"
-#include "rtio.h"
-#include "i2c.h"
-
-
-static void i2c_halfperiod()
-{
-    timer_kernel_en_write(0);
-    timer_kernel_load_write(CONFIG_CLOCK_FREQUENCY/10000);
-    timer_kernel_reload_write(0);
-    timer_kernel_en_write(1);
-
-    timer_kernel_update_value_write(1);
-    while(timer_kernel_value_read() != 0)
-        timer_kernel_update_value_write(1);
-}
-
-#if (defined CONFIG_I2C_BUS_COUNT) && (CONFIG_I2C_BUS_COUNT > 0)
-
-#define SDA_BIT (1 << (2*busno + 1))
-#define SCL_BIT (1 << (2*busno))
-
-static int i2c_sda_i(int busno)
-{
-    if(busno >= CONFIG_I2C_BUS_COUNT)
-        return 1;
-    else
-        return i2c_in_read() & SDA_BIT;
-}
-
-static void i2c_sda_oe(int busno, int oe)
-{
-    int reg;
-
-    reg = i2c_oe_read();
-    if(oe)
-        reg |= SDA_BIT;
-    else
-        reg &= ~SDA_BIT;
-    i2c_oe_write(reg);
-}
-
-static void i2c_sda_o(int busno, int o)
-{
-    int reg;
-
-    reg = i2c_out_read();
-    if(o)
-        reg |= SDA_BIT;
-    else
-        reg &= ~SDA_BIT;
-    i2c_out_write(reg);
-}
-
-static void i2c_scl_oe(int busno, int oe)
-{
-    int reg;
-
-    reg = i2c_oe_read();
-    if(oe)
-        reg |= SCL_BIT;
-    else
-        reg &= ~SCL_BIT;
-    i2c_oe_write(reg);
-}
-
-static void i2c_scl_o(int busno, int o)
-{
-    int reg;
-
-    reg = i2c_out_read();
-    if(o)
-        reg |= SCL_BIT;
-    else
-        reg &= ~SCL_BIT;
-    i2c_out_write(reg);
-}
-
-#else
-
-static int i2c_sda_i(int busno)
-{
-    return 1;
-}
-static void i2c_sda_oe(int busno, int oe) {}
-static void i2c_sda_o(int busno, int o) {}
-static void i2c_scl_oe(int busno, int oe) {}
-static void i2c_scl_o(int busno, int o) {}
-
-#endif
-
-
-void i2c_init(int busno)
-{
-    /* Set SCL as output, and high level */
-    i2c_scl_o(busno, 1);
-    i2c_scl_oe(busno, 1);
-    /* Prepare a zero level on SDA so that i2c_sda_oe pulls it down */
-    i2c_sda_o(busno, 0);
-    /* Release SDA */
-    i2c_sda_oe(busno, 0);
-
-    /* Check the I2C bus is ready */
-    i2c_halfperiod();
-    i2c_halfperiod();
-    if(!i2c_sda_i(busno))
-        artiq_raise_from_c("I2CError", "SDA is stuck low", 0, 0, 0);
-}
-
-void i2c_start(int busno)
-{
-    /* Set SCL high then SDA low */
-    i2c_scl_o(busno, 1);
-    i2c_halfperiod();
-    i2c_sda_oe(busno, 1);
-    i2c_halfperiod();
-}
-
-void i2c_stop(int busno)
-{
-    /* First, make sure SCL is low, so that the target releases the SDA line */
-    i2c_scl_o(busno, 0);
-    i2c_halfperiod();
-    /* Set SCL high then SDA high */
-    i2c_sda_oe(busno, 1);
-    i2c_scl_o(busno, 1);
-    i2c_halfperiod();
-    i2c_sda_oe(busno, 0);
-    i2c_halfperiod();
-}
-
-int i2c_write(int busno, int b)
-{
-    int i;
-
-    /* MSB first */
-    for(i=7;i>=0;i--) {
-        /* Set SCL low and set our bit on SDA */
-        i2c_scl_o(busno, 0);
-        i2c_sda_oe(busno, b & (1 << i) ? 0 : 1);
-        i2c_halfperiod();
-        /* Set SCL high ; data is shifted on the rising edge of SCL */
-        i2c_scl_o(busno, 1);
-        i2c_halfperiod();
-    }
-    /* Check ack */
-    /* Set SCL low, then release SDA so that the I2C target can respond */
-    i2c_scl_o(busno, 0);
-    i2c_halfperiod();
-    i2c_sda_oe(busno, 0);
-    /* Set SCL high and check for ack */
-    i2c_scl_o(busno, 1);
-    i2c_halfperiod();
-    /* returns 1 if acked (I2C target pulled SDA low) */
-    return !i2c_sda_i(busno);
-}
-
-int i2c_read(int busno, int ack)
-{
-    int i;
-    unsigned char b;
-
-    /* Set SCL low first, otherwise setting SDA as input may cause a transition
-     * on SDA with SCL high which will be interpreted as START/STOP condition.
-     */
-    i2c_scl_o(busno, 0);
-    i2c_halfperiod(); /* make sure SCL has settled low */
-    i2c_sda_oe(busno, 0);
-
-    b = 0;
-    /* MSB first */
-    for(i=7;i>=0;i--) {
-        i2c_scl_o(busno, 0);
-        i2c_halfperiod();
-        /* Set SCL high and shift data */
-        i2c_scl_o(busno, 1);
-        i2c_halfperiod();
-        if(i2c_sda_i(busno)) b |= (1 << i);
-    }
-    /* Send ack */
-    /* Set SCL low and pull SDA low when acking */
-    i2c_scl_o(busno, 0);
-    if(ack)
-        i2c_sda_oe(busno, 1);
-    i2c_halfperiod();
-    /* then set SCL high */
-    i2c_scl_o(busno, 1);
-    i2c_halfperiod();
-
-    return b;
-}
diff --git a/artiq/runtime/i2c.h b/artiq/runtime/i2c.h
deleted file mode 100644
index c7aab6e22..000000000
--- a/artiq/runtime/i2c.h
+++ /dev/null
@@ -1,10 +0,0 @@
-#ifndef __I2C_H
-#define __I2C_H
-
-void i2c_init(int busno);
-void i2c_start(int busno);
-void i2c_stop(int busno);
-int i2c_write(int busno, int b);
-int i2c_read(int busno, int ack);
-
-#endif