diff --git a/boot.c b/boot.c
index 4c188ad8..3646a0a0 100644
--- a/boot.c
+++ b/boot.c
@@ -398,8 +398,8 @@ void setup_i2c(void) __banked
 	reg_read_m(RTL837X_PIN_MUX_1);
 	for (uint8_t sfp = 0; sfp < machine.n_sfp; sfp++) {
 		uint8_t i2c = machine.sfp_port[sfp].i2c;
-		uint8_t scl_bus = (i2c >> 5) & 0x03;
-		uint8_t sda_bus = (i2c >> 2) & 0x07;
+		uint8_t scl_bus = (i2c >> RTL837X_REG_I2C_SCL_SHIFT) & 0x03;
+		uint8_t sda_bus = (i2c >> RTL837X_REG_I2C_SDA_SHIFT) & 0x07;
 		print_string("Configuring I2C for SFP idx="); print_byte(sfp); print_string(" SCL="); print_byte(scl_bus); print_string(", SDA="); print_byte(sda_bus); write_char('\n');
 		switch (scl_bus) {
 			case 3:
diff --git a/machine.c b/machine.c
index a4f8e192..9ce62920 100644
--- a/machine.c
+++ b/machine.c
@@ -6,8 +6,8 @@
 #include "rtl837x_common.h"
 
 #define I2CBUS(sda, scl) \
-    ((uint8_t)((I2C_BUS_FROM_SCL_PIN(scl) << 5) | \
-                     (I2C_BUS_FROM_SDA_PIN(sda) << 2)))
+    ((uint8_t)((I2C_BUS_FROM_SCL_PIN(scl) << RTL837X_REG_I2C_SCL_SHIFT) | \
+                     (I2C_BUS_FROM_SDA_PIN(sda) << RTL837X_REG_I2C_SDA_SHIFT)))
 
 #define I2C_BUS_FROM_SDA_PIN(sda_pin) \
 	((sda_pin) == GPIO47_I2C_SDA0 ? (uint8_t)0 : \
diff --git a/rtl837x_regs.h b/rtl837x_regs.h
index 6154221b..435c0c2d 100644
--- a/rtl837x_regs.h
+++ b/rtl837x_regs.h
@@ -126,6 +126,8 @@
 #define RTL837X_REG_I2C_CTRL2		0x041c
 #define RTL837X_REG_I2C_IN		0x0420
 #define RTL837X_REG_I2C_OUT		0x0424
+#define RTL837X_REG_I2C_SCL_SHIFT 5
+#define RTL837X_REG_I2C_SDA_SHIFT 2
 
 /*
  * NIC Related registers