+# define I2C_HIGH_SLOW 5000 /* ns, 100kHz clock */
+#ifdef TELEMEGA
+# define I2C_HIGH_FAST 2000 /* ns, 167kHz clock */
+#else
+# define I2C_HIGH_FAST 1000 /* ns, 333kHz clock */
+#endif
+
+# define I2C_RISE_SLOW 500 /* ns */
+# define I2C_RISE_FAST 100 /* ns */
+
+/* Clock period in ns */
+#define CYCLES(period) (((period) * (AO_PCLK1 / 1000)) / 1000000)
+
+#define max(a,b) ((a) > (b) ? (a) : (b))
+#define I2C_CCR_HIGH_SLOW max(4,CYCLES(I2C_HIGH_SLOW))
+#define I2C_CCR_HIGH_FAST max(4,CYCLES(I2C_HIGH_FAST))
+#define I2C_TRISE_SLOW (CYCLES(I2C_RISE_SLOW) + 1)
+#define I2C_TRISE_FAST (CYCLES(I2C_RISE_FAST) + 1)
+
+#if I2C_FAST
+#define I2C_TRISE I2C_TRISE_FAST
+#define I2C_CCR_HIGH I2C_CCR_HIGH_FAST
+#else
+#define I2C_TRISE I2C_TRISE_SLOW
+#define I2C_CCR_HIGH I2C_CCR_HIGH_SLOW
+#endif
+