diff --git a/inc/system/gpio.h b/inc/system/gpio.h
--- a/inc/system/gpio.h
+++ b/inc/system/gpio.h
@@ -4,6 +4,12 @@
 #include "stm32f3xx_hal.h"
 
 
+// Helper macros
+#define CHANGE_PERIOD_MS 100
+#define CHANGE_ELAPSED (HAL_GetTick() - change_time_reset) > CHANGE_PERIOD_MS
+#define CHANGE_RESET change_time_reset = HAL_GetTick()
+
+
 // Define button push
 #define SW_D_Pin GPIO_PIN_4
 #define SW_D_GPIO_Port GPIOA
diff --git a/lib/ssd1306/ssd1306.c b/lib/ssd1306/ssd1306.c
--- a/lib/ssd1306/ssd1306.c
+++ b/lib/ssd1306/ssd1306.c
@@ -1,34 +1,21 @@
+//
+// ssd1306: OLED display driver
+//
+
 #include "stm32f3xx_hal.h"
 #include "ssd1306.h"
 
 
-SPI_HandleTypeDef hspi1;
-
-
-// SPI handle accessor
-SPI_HandleTypeDef* spi_get()
-{
-    return &hspi1;
-}
+// Private variables
+static SPI_HandleTypeDef hspi1;
 
 
-// Write command to OLED
-static void WriteCommand(unsigned char command)
-{
-  SSD_CS_Low();
-  SSD_A0_Low();
-  SPI_SendByte(command);
-  SSD_CS_High();
-}
+// Private method prototypes
+static void WriteCommand(unsigned char command);
+static void WriteData(unsigned char data);
+static void setStartPage(unsigned char d);
+static void setStartColumn(unsigned char d);
 
-// Write data to OLED
-static void WriteData(unsigned char data)
-{
-  SSD_CS_Low();
-  SSD_A0_High();
-  SPI_SendByte(data);
-  SSD_CS_High();
-}
 
 // Initialize OLED
 void ssd1306_init(void)
@@ -38,7 +25,7 @@ void ssd1306_init(void)
 	__GPIOB_CLK_ENABLE();
 	GPIO_InitTypeDef GPIO_InitStruct;
 
-	/*Configure GPIO pins : OLED_CS_Pin OLED_RESET_Pin OLED_DC_Pin */
+	// GPIO
 	GPIO_InitStruct.Pin = OLED_CS_Pin|OLED_RESET_Pin|OLED_DC_Pin;
 	GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
 	GPIO_InitStruct.Pull = GPIO_NOPULL;
@@ -227,10 +214,27 @@ static const char fontData[][5] =
 	{0x22,0x5d,0x22,0x00,0x00},			//   (111) ez updown
 	{0x14,0x3e,0x14,0x00,0x00},         //   (112) ez updown short
 };
-/*
 
 
-*/
+// Write command to OLED
+static void WriteCommand(unsigned char command)
+{
+  SSD_CS_Low();
+  SSD_A0_Low();
+  SPI_SendByte(command);
+  SSD_CS_High();
+}
+
+
+// Write data to OLED
+static void WriteData(unsigned char data)
+{
+  SSD_CS_Low();
+  SSD_A0_High();
+  SPI_SendByte(data);
+  SSD_CS_High();
+}
+
 
 // Set start page
 static void setStartPage(unsigned char d)
@@ -248,7 +252,6 @@ static void setStartColumn(unsigned char
                                 // Default => 0x10
 }
 
-
 // Therm logo
 const uint8_t row[4][32] = {
 
@@ -439,4 +442,10 @@ void ssd1306_drawstringbig(const unsigne
 }
 
 
+// SPI handle accessor
+SPI_HandleTypeDef* spi_get()
+{
+    return &hspi1;
+}
+
 // vim:softtabstop=4 shiftwidth=4 expandtab
diff --git a/src/pid.c b/src/pid.c
--- a/src/pid.c
+++ b/src/pid.c
@@ -5,7 +5,7 @@
 #include "pid.h"
 #include "flash.h"
 
-// PID implementation
+// Private variables
 static pid_state_t state;
 
 
diff --git a/src/pwmout.c b/src/pwmout.c
--- a/src/pwmout.c
+++ b/src/pwmout.c
@@ -8,11 +8,13 @@
 #include "flash.h"
 #include "error.h"
 
-TIM_HandleTypeDef htim17;
 
+// Private variables
+static TIM_HandleTypeDef htim17;
 static uint32_t last_ssr_on = 0;
 
 
+// Initialize hardware PWM output
 void pwmout_init(void)
 {
 	GPIO_InitTypeDef GPIO_InitStruct;
@@ -100,6 +102,8 @@ void pwmout_process(int16_t duty)
 	htim17.Instance->CCR1 = duty; //duty;
 }
 
+
+// Accessor for timer handle
 TIM_HandleTypeDef* pwmout_get_tim(void)
 {
 	return &htim17;
diff --git a/src/system/flash.c b/src/system/flash.c
--- a/src/system/flash.c
+++ b/src/system/flash.c
@@ -6,11 +6,12 @@
 #include "flash.h"
 #include "config.h"
 
-// Takes up 1 page (1k size)
-static __attribute__((__section__(".eeprom"))) uint16_t eeprom_emulation[512];
 
-therm_settings_t settings;
-therm_status_t status;
+// Private variables
+static __attribute__((__section__(".eeprom"))) uint16_t eeprom_emulation[512]; // Takes up 1 page (1k size)
+static therm_settings_t settings;
+static therm_status_t status;
+
 
 // Initialize flash and restore settings
 void flash_init(void)
diff --git a/src/system/gpio.c b/src/system/gpio.c
--- a/src/system/gpio.c
+++ b/src/system/gpio.c
@@ -3,50 +3,12 @@
 //
 
 #include "stm32f3xx_hal.h"
-
 #include "gpio.h"
 
-#define CHANGE_PERIOD_MS 100
-#define CHANGE_ELAPSED (HAL_GetTick() - change_time_reset) > CHANGE_PERIOD_MS
-#define CHANGE_RESET change_time_reset = HAL_GetTick()
-
-// Increase on each press, and increase at a fast rate after duration elapsed of continuously holding down... somehow...
-static uint32_t change_time_reset = 0;
-
 
-// Increment/decrement unsigned variable with up/down buttons
-void user_input(uint16_t* to_modify)
-{
-    if(CHANGE_ELAPSED) {
-        if(!HAL_GPIO_ReadPin(SW_UP) ) {
-            CHANGE_RESET;
-            (*to_modify)++;
-        }
-        else if(!HAL_GPIO_ReadPin(SW_DOWN) && (*to_modify) > 0) {
-            CHANGE_RESET;
-            (*to_modify)--;
-        }
-    }
-}
-
-
-// Increment/decrement signed variable with up/down buttons
-void user_input_signed(int32_t* to_modify)
-{
-	//fixme: need to cast to 16/32 bits correctly
-    if(CHANGE_ELAPSED) {
-        if(!HAL_GPIO_ReadPin(SW_UP) ) {
-            CHANGE_RESET;
-            if (*to_modify < 32768)
-            	(*to_modify)++;
-        }
-        else if(!HAL_GPIO_ReadPin(SW_DOWN)) {
-            CHANGE_RESET;
-            if (*to_modify >= -32768)
-            	(*to_modify)--;
-        }
-    }
-}
+// Private variables
+// Increase on each press, and increase at a fast rate after duration elapsed of continuously holding down... somehow...
+static uint32_t change_time_reset = 0;
 
 
 // Initialize GPIOs
@@ -90,3 +52,38 @@ void gpio_init(void)
 	HAL_GPIO_WritePin(LED, 0);
 
 }
+
+
+// Increment/decrement unsigned variable with up/down buttons
+void user_input(uint16_t* to_modify)
+{
+    if(CHANGE_ELAPSED) {
+        if(!HAL_GPIO_ReadPin(SW_UP) ) {
+            CHANGE_RESET;
+            (*to_modify)++;
+        }
+        else if(!HAL_GPIO_ReadPin(SW_DOWN) && (*to_modify) > 0) {
+            CHANGE_RESET;
+            (*to_modify)--;
+        }
+    }
+}
+
+
+// Increment/decrement signed variable with up/down buttons
+void user_input_signed(int32_t* to_modify)
+{
+	//fixme: need to cast to 16/32 bits correctly
+    if(CHANGE_ELAPSED) {
+        if(!HAL_GPIO_ReadPin(SW_UP) ) {
+            CHANGE_RESET;
+            if (*to_modify < 32768)
+            	(*to_modify)++;
+        }
+        else if(!HAL_GPIO_ReadPin(SW_DOWN)) {
+            CHANGE_RESET;
+            if (*to_modify >= -32768)
+            	(*to_modify)--;
+        }
+    }
+}
diff --git a/src/tempsense.c b/src/tempsense.c
--- a/src/tempsense.c
+++ b/src/tempsense.c
@@ -18,8 +18,6 @@ void tempsense_init(void)
 	// Maybe don't perform temp sensor setup in here, but in readtemp?
 	// what happens if the user changes the tempsense type while running?
 	// we need to re-init...
-
-
 }