Display: create and use display_set_cursor().

Having this in a seperate function will be helpful later.

display.h

@@ -34,6 +34,8 @@
 
 void display_init(void);
 void display_clear(void);
-void display_text_P(uint8_t line, uint8_t column, PGM_P message_P);
+
+void display_set_cursor(uint8_t line, uint8_t column);
+void display_text_P(PGM_P message_P);
 
 #endif /* _DISPLAY_H */

display_ssd1306.c

@@ -78,10 +78,16 @@ void display_clear(void) {
 }
 
 /**
-  Prints the text at a given position.
+  Sets the cursor to the given position.
+
+  \param line   The vertical cursor position to set, in lines. First line is
+                zero. Line height is character height, which is currently
+                fixed to 8 pixels.
+
+  \param column The horizontal cursor position to set, in pixels. First
+                column is zero.
 */
-void display_text_P(uint8_t line, uint8_t column, PGM_P message) {
+void display_set_cursor(uint8_t line, uint8_t column) {
-  uint8_t i, index;
 
   // Enter command mode.
   displaybus_write(0x00, 0);
@@ -90,6 +96,16 @@ void display_text_P(uint8_t line, uint8_t column, PGM_P message) {
   // Set column.
   displaybus_write(0x00 | (column & 0x0F), 0);
   displaybus_write(0x10 | ((column >> 4) & 0x0F), 1);
+}
+
+/**
+  Prints the text at the current cursor position.
+
+  \param message Zero terminated string of the text to be displayed, stored
+                 in program memory.
+*/
+void display_text_P(PGM_P message) {
+  uint8_t i, index;
 
   // Render text to bitmap to display.
   displaybus_write(0x40, 0);

i2c_test.c

@@ -24,7 +24,8 @@ static void i2c_test(void) {
     "Welcome to Teacup" is 64 pixel columns wide, entire display is
     128 columns, so we offset by 32 columns to get it to the center.
   */
-  display_text_P(1, 32, PSTR("Welcome to Teacup"));
+  display_set_cursor(1, 32);
+  display_text_P(PSTR("Welcome to Teacup"));
 }
 
 #endif /* I2C_TEST */