diff --git a/lib/card10/portexpander.c b/lib/card10/portexpander.c
index 920d8dfa9d365e15e48ee3d956a106816e28bfa0..ee3ac3da9df23430800a7892bbfcd326232d5e3a 100644
--- a/lib/card10/portexpander.c
+++ b/lib/card10/portexpander.c
@@ -7,17 +7,57 @@
 #include <string.h>
 #include <stdbool.h>
 
+// PCAL6408A I2C port expander
+
 static bool detected = false;
 static uint8_t output_state;
 
+/* clang-format off */
+#define PE_ADDR 0x42
+
+#define PE_C_INPUT_PORT         0x00
+#define PE_C_OUTPUT_PORT        0x01
+#define PE_C_POLARITY_INV       0x02
+#define PE_C_CONFIG             0x03
+#define PE_C_OUTPUT_DRIVE_STR_0 0x40
+#define PE_C_OUTPUT_DRIVE_STR_1 0x41
+#define PE_C_INPUT_LATCH        0x42
+#define PE_C_PULL_ENABLE        0x43
+#define PE_C_PULL_SEL           0x44
+#define PE_C_INT_MASK           0x45
+#define PE_C_INT_STATUS         0x46
+#define PE_C_OUTPUT_PORT_CONFIG 0x4F
+
+#define PE_CONFIG_OUT 0
+#define PE_CONFIG_IN  1
+
+#define PE_PULL_DOWN 0
+#define PE_PULL_UP   1
+
+#define PE_OUT_PUSH_PULL  0
+#define PE_OUT_OPEN_DRAIN 1
+/* clang-format on */
+
+#define PE_INPUT_MASK ((uint8_t)0b01101000) // 3, 5, 6 = input
+
+static int portexpander_write(uint8_t command, uint8_t data)
+{
+	uint8_t i2c_data[2] = { command, data };
+	return I2C_MasterWrite(MXC_I2C1_BUS0, PE_ADDR, i2c_data, 2, 0);
+}
+
+static int portexpander_read(uint8_t command, uint8_t *data)
+{
+	I2C_MasterWrite(MXC_I2C1_BUS0, PE_ADDR, &command, 1, 1);
+	return I2C_MasterRead(MXC_I2C1_BUS0, PE_ADDR, data, 1, 0);
+}
+
 void portexpander_init(void)
 {
-	uint8_t addr = 0x21;
 	int ret;
 
-	// Enable pull-ups for buttons
-	uint8_t command[] = { 0x43, 0x68 };
-	ret = I2C_MasterWrite(MXC_I2C1_BUS0, addr << 1, command, 2, 0);
+	// Enable pull-ups for buttons (type defaults to pull-up)
+	ret = portexpander_write(PE_C_PULL_ENABLE, PE_INPUT_MASK);
 
 	if (ret != 2) {
 		printf("portexpander NOT detected\n");
@@ -27,31 +67,22 @@ void portexpander_init(void)
 	detected = true;
 
 	// Set _all_ outputs to open-drain to support the high side p-channel transistors.
-	command[0] = 0x4F;
-	command[1] = 0x01;
-	I2C_MasterWrite(MXC_I2C1_BUS0, addr << 1, command, 2, 0);
+	portexpander_write(PE_C_OUTPUT_PORT_CONFIG, PE_OUT_OPEN_DRAIN);
 
 	// Enable outputs for the transistors, the LED and the LCD reset
-	command[0] = 0x03;
-	command[1] = 0x68;
-	I2C_MasterWrite(MXC_I2C1_BUS0, addr << 1, command, 2, 0);
+	portexpander_write(PE_C_CONFIG, PE_INPUT_MASK);
 
 	// Set outputs to high (i.e. open-drain)
-	output_state = 0x97;
-	command[0]   = 0x01;
-	command[1]   = output_state;
-	I2C_MasterWrite(MXC_I2C1_BUS0, addr << 1, command, 2, 0);
+	output_state = ~PE_INPUT_MASK;
+	portexpander_write(PE_C_OUTPUT_PORT, output_state);
 }
 
 uint8_t portexpander_get(void)
 {
-	uint8_t addr      = 0x21;
-	uint8_t command[] = { 0x00 };
-	uint8_t buf       = 0xFF;
+	uint8_t buf = 0xFF;
 
 	if (detected) {
-		I2C_MasterWrite(MXC_I2C1_BUS0, addr << 1, command, 1, 1);
-		I2C_MasterRead(MXC_I2C1_BUS0, addr << 1, &buf, 1, 0);
+		portexpander_read(PE_C_INPUT_PORT, &buf);
 	}
 
 	return buf;
@@ -64,9 +95,6 @@ bool portexpander_detected(void)
 
 void portexpander_set(uint8_t pin, uint8_t value)
 {
-	uint8_t addr = 0x21;
-	uint8_t command[2];
-
 	if (detected && pin < 8) {
 		if (value) {
 			output_state |= (1 << pin);
@@ -74,8 +102,6 @@ void portexpander_set(uint8_t pin, uint8_t value)
 			output_state &= ~(1 << pin);
 		}
 
-		command[0] = 0x01;
-		command[1] = output_state;
-		I2C_MasterWrite(MXC_I2C1_BUS0, addr << 1, command, 2, 0);
+		portexpander_write(PE_C_OUTPUT_PORT, output_state);
 	}
 }