Display: queue up display clear commands.

display.h

@@ -41,6 +41,22 @@
 
 #define DISPLAY_BUFFER_SIZE 128
 
+/**
+  Printable ASCII characters and our embedded fonts start at 0x20, so we can
+  use 0x00..0x1F for storing control commands in the character queue. That's
+  what genuine ASCII does, too, we just use our own code set.
+
+  Queueing up actions together with actual characters not only postpones
+  these actions to idle time, it's also necessary to keep them in the right
+  order. Without it, writing a few characters, moving the cursor elsewhere and
+  writing even more characters would result in all characters being written to
+  the second position, because characters would wait in the queue while cursor
+  movements were executed immediately.
+*/
+enum display_low_code {
+  low_code_clear = 0x01,
+  low_code_set_cursor
+};
 
 void display_init(void);
 void display_tick(void);

display_hd44780.c

@@ -7,19 +7,7 @@
 /**
   TODO list:
 
-    - Procedures like display_clear() and display_set_cursor() should be queued
+    - Implement display_set_cursor().
-      up, too. Just like characters. Fonts start at 0x20, so 0x00..0x1F are
-      available for command sequences. For example, setting the cursor could
-      queue up 0x04 0x01 0x20 (3 bytes) to set the cursor to line 1, column 32.
-      0x04 is the "command", bytes are queued up with display_writechar().
-
-      This is necessary to enforce characters and cursor commands to happen in
-      the right order. Currently, writing a few characters, moving the cursor
-      elsewhere and writing even more characters results in all characters
-      being written to the second position, because characters wait in the
-      queue, while cursor movements are executed immediately.
-
-      Code currently in display_set_cursor() would move to display_tick(), then.
 */
 
 #include "display.h"
@@ -50,10 +38,10 @@ void display_init(void) {
 }
 
 /**
-  Clear the screen. This display has a dedicated command for doing it.
+  Queue up a clear screen command. Cheap operation on this display.
 */
 void display_clear(void) {
-  displaybus_write(0x01, parallel_4bit_instruction);
+  display_writechar((uint8_t)low_code_clear);
 }
 
 /**
@@ -105,8 +93,8 @@ void display_clock(void) {
 }
 
 /**
-  Forwards a character from the display queue to display bus. As this is a
+  Forwards a character or a control command from the display queue to display
-  character based display it's easy.
+  bus. As this is a character based display it's easy.
 */
 void display_tick() {
   uint8_t data;
@@ -117,7 +105,16 @@ void display_tick() {
 
   if (buf_canread(display)) {
     buf_pop(display, data);
-    displaybus_write(data, parallel_4bit_data);
+    switch (data) {
+      case low_code_clear:
+        displaybus_write(0x01, parallel_4bit_instruction);
+        break;
+
+      default:
+        // Should be a printable character.
+        displaybus_write(data, parallel_4bit_data);
+        break;
+    }
   }
 }
 

display_ssd1306.c

@@ -35,19 +35,7 @@
 
   TODO list:
 
-    - Procedures like display_clear() and display_set_cursor() should be queued
+    - Move code in display_set_cursor() would to display_tick().
-      up, too. Just like characters. Fonts start at 0x20, so 0x00..0x1F are
-      available for command sequences. For example, setting the cursor could
-      queue up 0x04 0x01 0x20 (3 bytes) to set the cursor to line 1, column 32.
-      0x04 is the "command", bytes are queued up with display_writechar().
-
-      This is necessary to enforce characters and cursor commands to happen in
-      the right order. Currently, writing a few characters, moving the cursor
-      elsewhere and writing even more characters results in all characters
-      being written to the second position, because characters wait in the
-      queue, while cursor movements are executed immediately.
-
-      Code currently in display_set_cursor() would move to display_tick(), then.
 
     - Lot's of prettification. Like a nice background picture with the Teacup
       logo, like "Welcome to Teacup" as a greeting screen, like writing numbers
@@ -127,28 +115,11 @@ void display_init(void) {
 }
 
 /**
-  Clear the screen. As this display supports many sophisticated commands,
+  Queue up a clear screen command. Be careful, this is an expensive operation
-  but not a simple 'clear', we have to overwrite the entire memory with
+  on this display.
-  zeros, byte by byte.
 */
 void display_clear(void) {
-  uint16_t i;
+  display_writechar((uint8_t)low_code_clear);
-
-  // Set horizontal adressing mode.
-  displaybus_write(0x00, 0);
-  displaybus_write(0x20, 0);
-  displaybus_write(0x00, 1);
-
-  // Write 512 zeros.
-  displaybus_write(0x40, 0);
-  for (i = 0; i < 512; i++) {
-    displaybus_write(0x00, (i == 511));
-  }
-
-  // Return to page adressing mode.
-  displaybus_write(0x00, 0);
-  displaybus_write(0x20, 0);
-  displaybus_write(0x02, 1);
 }
 
 /**
@@ -212,10 +183,11 @@ void display_clock(void) {
 }
 
 /**
-  Forwards a character from the display queue to the I2C queue.
+  Forwards a character or a control command from the display queue to the I2C
+  queue.
 */
 void display_tick() {
-  uint8_t i, data, index;
+  uint16_t i, data, index;
 
   if (displaybus_busy()) {
     return;
@@ -232,22 +204,50 @@ void display_tick() {
 
   if (buf_canread(display)) {
     buf_pop(display, data);
-    index = data - 0x20;
+    switch (data) {
+      case low_code_clear:
+        /**
+          Clear the screen. As this display supports many sophisticated
+          commands, but not a simple 'clear', we have to overwrite the entire
+          memory with zeros, byte by byte.
+        */
+        // Set horizontal adressing mode.
+        displaybus_write(0x00, 0);
+        displaybus_write(0x20, 0);
+        displaybus_write(0x00, 1);
 
-    // Write pixels command.
+        // Write 512 zeros.
-    displaybus_write(0x40, 0);
+        displaybus_write(0x40, 0);
+        for (i = 0; i < 512; i++) {
+          displaybus_write(0x00, (i == 511));
+        }
 
-    // Send the character bitmap.
+        // Return to page adressing mode.
-    #ifdef FONT_IS_PROPORTIONAL
+        displaybus_write(0x00, 0);
-      for (i = 0; i < pgm_read_byte(&font[index].columns); i++) {
+        displaybus_write(0x20, 0);
-    #else
+        displaybus_write(0x02, 1);
-      for (i = 0; i < FONT_COLUMNS; i++) {
+        break;
-    #endif
+
-        displaybus_write(pgm_read_byte(&font[index].data[i]), 0);
+      default:
-    }
+        // Should be a printable character.
-    // Send space between characters.
+        index = data - 0x20;
-    for (i = 0; i < FONT_SYMBOL_SPACE; i++) {
+
-      displaybus_write(0x00, (i == FONT_SYMBOL_SPACE - 1));
+        // Write pixels command.
+        displaybus_write(0x40, 0);
+
+        // Send the character bitmap.
+        #ifdef FONT_IS_PROPORTIONAL
+          for (i = 0; i < pgm_read_byte(&font[index].columns); i++) {
+        #else
+          for (i = 0; i < FONT_COLUMNS; i++) {
+        #endif
+            displaybus_write(pgm_read_byte(&font[index].data[i]), 0);
+        }
+        // Send space between characters.
+        for (i = 0; i < FONT_SYMBOL_SPACE; i++) {
+          displaybus_write(0x00, (i == FONT_SYMBOL_SPACE - 1));
+        }
+        break;
     }
   }
 }