From aab447567cc9962ca4f7ae9b0d0260df969a5984 Mon Sep 17 00:00:00 2001
From: Markus Hitter <mah@jump-ing.de>
Date: Tue, 5 Apr 2016 18:38:07 +0200
Subject: [PATCH] I2C: rewrite demo to use program memory.

Advantages are apparent: much less RAM usage.

$ avr-size -C --mcu=atmega1284p build/teacup.elf
AVR Memory Usage
----------------
Device: atmega1284p

Program:   23750 bytes (18.1% Full)
(.text + .data + .bootloader)

Data:       2053 bytes (12.5% Full)
(.data + .bss + .noinit)

EEPROM:       32 bytes (0.8% Full)
(.eeprom)

$ avr-size -C --mcu=atmega1284p build/teacup.elf
AVR Memory Usage
----------------
Device: atmega1284p

Program:   23720 bytes (18.1% Full)
(.text + .data + .bootloader)

Data:       1543 bytes (9.4% Full)
(.data + .bss + .noinit)

EEPROM:       32 bytes (0.8% Full)
(.eeprom)

A note about size reports: what 'make size' currently reports is
a bit misleading, because it doesn't count the .data section as
Flash usage. However, this section is actually written there as
it's the section which initialises RAM.
---
 i2c_test.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/i2c_test.c b/i2c_test.c
index 63a5e8b..c7ffd4a 100644
--- a/i2c_test.c
+++ b/i2c_test.c
@@ -20,7 +20,7 @@
 
 #define DISPLAY_I2C_ADDRESS        (0x3C << 1)
 
-static uint8_t display_init[] = {
+const uint8_t PROGMEM display_init[] = {
   0x00,             // Command marker.
   0xAE,             // Display off.
   0xD5, 0x80,       // Display clock divider (reset).
@@ -50,7 +50,7 @@ typedef struct {
   uint8_t data[4];
 } SYMBOL;
 
-SYMBOL font_8x4[] = {
+const SYMBOL PROGMEM font_8x4[] = {
   {2, {0x00, 0x00, 0x00, 0x00}},  /* space */
   {3, {0x0C, 0x5E, 0x0C, 0x00}},  /* excl_mark */
   {3, {0x03, 0x00, 0x03, 0x00}},  /* quot_mark */
@@ -162,7 +162,7 @@ static void i2c_test(void) {
 
   for (i = 0; i < sizeof(display_init); i++) {
     // Send last byte with 'last_byte' set.
-    i2c_write(DISPLAY_I2C_ADDRESS, display_init[i],
+    i2c_write(DISPLAY_I2C_ADDRESS, pgm_read_byte(&display_init[i]),
               (i == sizeof(display_init) - 1));
   }
 
@@ -203,11 +203,12 @@ static void i2c_test(void) {
   // Render text to bitmap to display.
   i2c_write(DISPLAY_I2C_ADDRESS, 0x40, 0);
   while (*message) {
-    SYMBOL symbol = font_8x4[(uint8_t)*message - 0x20];
+    uint8_t index = (uint8_t)*message - 0x20;
 
     // Send the character bitmap.
-    for (i = 0; i < symbol.columns; i++) {
-      i2c_write(DISPLAY_I2C_ADDRESS, symbol.data[i], 0);
+    for (i = 0; i < pgm_read_byte(&font_8x4[index].columns); i++) {
+      i2c_write(DISPLAY_I2C_ADDRESS,
+                pgm_read_byte(&font_8x4[index].data[i]), 0);
     }
     // Send space between characters.
     for (i = 0; i < FONT_SYMBOLS_SPACE; i++) {