i2c.c: simplify error handling.

If all error conditions are handled the same, there's not much
point to use distinct code for each of them.

Also, handle collisions like the other error conditions.

This saves a nice 52 bytes of program memory.

  Program:  24404 bytes
     Data:   1543 bytes
   EEPROM:     32 bytes

i2c.c

@@ -65,9 +65,7 @@
 // Transmission not interrupted.
 #define I2C_NOINTERRUPTED     0b01111111
 
-#define I2C_ERROR_BUS_FAIL    0b00000001
+#define I2C_ERROR             0b00000001
-#define I2C_ERROR_NACK        0b00000010
-#define I2C_ERROR_NO_ANSWER   0b00010000
 #define I2C_ERROR_LOW_PRIO    0b00100000
 
 
@@ -214,9 +212,7 @@ void i2c_write(uint8_t data, uint8_t last_byte) {
   }
 
   // Recover from error conditions by draining the buffer.
-  if ((i2c_state & I2C_ERROR_BUS_FAIL) ||
+  if (i2c_state & I2C_ERROR) {
-      (i2c_state & I2C_ERROR_NO_ANSWER) ||
-      (i2c_state & I2C_ERROR_NACK)) {
     while (buf_canread(send)) {
       buf_pop(send, TWDR);
     }
@@ -278,21 +274,10 @@ ISR(TWI_vect) {
   #endif
 
   switch (status) {
-    case TW_BUS_ERROR:
-      // A hardware error was detected.
-      #ifdef TWI_INTERRUPT_DEBUG
-        serial_writechar('1');
-      #endif
-      i2c_state |= I2C_ERROR_BUS_FAIL;
-      // Let i2c_write() continue.
-      i2c_should_end = 0;
-      // Send stop condition.
-      TWCR = (1<<TWINT)|(I2C_MODE<<TWEA)|(0<<TWSTA)|(1<<TWSTO)|(1<<TWEN)|(0<<TWIE);
-      break;
     case TW_START:
       // Start happens, send a target address.
       #ifdef TWI_INTERRUPT_DEBUG
-        serial_writechar('2');
+        serial_writechar('1');
       #endif
       if ((i2c_state & I2C_MODE_MASK) == I2C_MODE_SARP) {
         i2c_address |= 0x01;
@@ -305,7 +290,7 @@ ISR(TWI_vect) {
     case TW_REP_START:
       // Start happens, send a target address.
       #ifdef TWI_INTERRUPT_DEBUG
-        serial_writechar('3');
+        serial_writechar('2');
       #endif
       if ((i2c_state & I2C_MODE_MASK) == I2C_MODE_ENHA) {
         i2c_address |= 0x01;
@@ -318,7 +303,7 @@ ISR(TWI_vect) {
     case TW_MT_SLA_ACK:
       // SLA+W was sent, then ACK received.
       #ifdef TWI_INTERRUPT_DEBUG
-        serial_writechar('4');
+        serial_writechar('3');
       #endif
       if ((i2c_state & I2C_MODE_MASK) == I2C_MODE_SAWP && buf_canread(send)) {
         buf_pop(send, TWDR);
@@ -331,21 +316,10 @@ ISR(TWI_vect) {
         }
       #endif
       break;
-    case TW_MT_SLA_NACK:
-      // SLA+W was sent, got NACK, so slave is busy or out of bus.
-      #ifdef TWI_INTERRUPT_DEBUG
-        serial_writechar('5');
-      #endif
-      i2c_state |= I2C_ERROR_NO_ANSWER;
-      // Let i2c_write() continue.
-      i2c_should_end = 0;
-      // Send stop condition.
-      TWCR = (1<<TWINT)|(I2C_MODE<<TWEA)|(0<<TWSTA)|(1<<TWSTO)|(1<<TWEN)|(0<<TWIE);
-      break;
     case TW_MT_DATA_ACK:
       // A byte was sent, got ACK.
       #ifdef TWI_INTERRUPT_DEBUG
-        serial_writechar('6');
+        serial_writechar('4');
       #endif
       if ((i2c_state & I2C_MODE_MASK) == I2C_MODE_SAWP) {
         if (buf_canread(send)) {
@@ -374,33 +348,6 @@ ISR(TWI_vect) {
         }
       #endif
       break;
-    case TW_MT_DATA_NACK:
-      // Byte was sent but got NACK, there are two possible reasons:
-      //  - a slave stops transmission and it is ok, or
-      //  - a slave became crazy.
-      #ifdef TWI_INTERRUPT_DEBUG
-        serial_writechar('7');
-      #endif
-      i2c_state |= I2C_ERROR_NACK;
-      // Let i2c_write() continue.
-      i2c_should_end = 0;
-      // Send stop condition.
-      TWCR = (1<<TWINT)|(I2C_MODE<<TWEA)|(0<<TWSTA)|(1<<TWSTO)|(1<<TWEN)|(0<<TWIE);
-      break;
-    case TW_MT_ARB_LOST:  // Collision, identical to TW_MR_ARB_LOST.
-      // It looks like there is another master on the bus.
-      #ifdef TWI_INTERRUPT_DEBUG
-        serial_writechar('8');
-      #endif
-      i2c_state |= I2C_ERROR_LOW_PRIO;
-      // Setup all again.
-      sendtail = sendhead;
-      #ifdef I2C_EEPROM_SUPPORT
-        i2c_page_index = 0;
-      #endif
-      // Try to resend when the bus became free.
-      TWCR = (1<<TWINT)|(I2C_MODE<<TWEA)|(1<<TWSTA)|(0<<TWSTO)|(1<<TWEN)|(1<<TWIE);
-      break;
 
     #ifdef I2C_READ_SUPPORT
 
@@ -534,6 +481,39 @@ ISR(TWI_vect) {
 
     #endif /* I2C_SLAVE_MODE */
 
+    case TW_BUS_ERROR:
+      // A hardware error was detected.
+      #ifdef TWI_INTERRUPT_DEBUG
+        serial_writechar('5');
+      #endif
+    case TW_MT_SLA_NACK:
+      // SLA+W was sent, got NACK, so slave is busy or out of bus.
+      #ifdef TWI_INTERRUPT_DEBUG
+        serial_writechar('6');
+      #endif
+    case TW_MT_DATA_NACK:
+      // Byte was sent but got NACK, there are two possible reasons:
+      //  - a slave stops transmission and it is ok, or
+      //  - a slave became crazy.
+      #ifdef TWI_INTERRUPT_DEBUG
+        serial_writechar('7');
+      #endif
+    case TW_MT_ARB_LOST:
+      // Collision, identical to TW_MR_ARB_LOST.
+      // It looks like there is another master on the bus. Handle this like
+      // the other error conditions, because an eventual resend is handled in
+      // upper layers (display code).
+      #ifdef TWI_INTERRUPT_DEBUG
+        serial_writechar('8');
+      #endif
+
+      i2c_state |= I2C_ERROR;
+      // Let i2c_write() continue.
+      i2c_should_end = 0;
+      // Send stop condition.
+      TWCR = (1<<TWINT)|(I2C_MODE<<TWEA)|(0<<TWSTA)|(1<<TWSTO)|(1<<TWEN)|(0<<TWIE);
+      break;
+
     default:
       #ifdef TWI_INTERRUPT_DEBUG
         sendf_P(serial_writechar, PSTR("(%sx)"), status);