Simplify ACK handling

TWEA bit in TWCR register needs to be cleared to send NACK.
Clear it when too many bytes were received, re-enable it after
when going idle.

Add missing state for prev. send NACK.
Catch all invalid states and reset bus.

TWINT bit needs to be set every time in ISR.

README.md

@@ -10,10 +10,10 @@ Currently the following AVR MCUs are supported:
 
 AVR MCU | Flash bytes used (.text + .data) | Bootloader region size
 --- | --- | ---
-atmega8 | 868 (0x364) | 512 words
-atmega88 | 920 (0x398) | 512 words
-atmega168 | 972 (0x3CC) | 512 words
-atmega328p | 972 (0x3CC) | 512 words
+atmega8 | 872 (0x368) | 512 words
+atmega88 | 918 (0x396) | 512 words
+atmega168 | 970 (0x3CA) | 512 words
+atmega328p | 970 (0x3CA) | 512 words
 (Compiled on Ubuntu 18.04 LTS (gcc 5.4.0 / avr-libc 2.0.0) with EEPROM and LED support)
 
 

main.c

@@ -358,6 +358,7 @@ static uint8_t TWI_data_read(uint8_t bcnt)
 ISR(TWI_vect)
 {
     static uint8_t bcnt;
+    uint8_t control = TWCR;
 
     switch (TWSR & 0xF8)
     {
@@ -365,18 +366,13 @@ ISR(TWI_vect)
         case 0x60:
             bcnt = 0;
             LED_RT_ON();
-            TWCR |= (1<<TWINT) | (1<<TWEA);
             break;
 
         /* prev. SLA+W, data received, ACK returned -> receive data and ACK */
         case 0x80:
-            if (TWI_data_write(bcnt++, TWDR))
+            if (TWI_data_write(bcnt++, TWDR) == 0x00)
             {
-                TWCR |= (1<<TWINT) | (1<<TWEA);
-            }
-            else
-            {
-                TWCR |= (1<<TWINT);
+                control &= ~(1<<TWEA);
                 bcnt = 0;
             }
             break;
@@ -389,22 +385,25 @@ ISR(TWI_vect)
         /* prev. SLA+R, data sent, ACK returned -> send data */
         case 0xB8:
             TWDR = TWI_data_read(bcnt++);
-            TWCR |= (1<<TWINT) | (1<<TWEA);
             break;
 
+        /* prev. SLA+W, data received, NACK returned -> IDLE */
+        case 0x88:
         /* STOP or repeated START -> IDLE */
         case 0xA0:
         /* prev. SLA+R, data sent, NACK returned -> IDLE */
         case 0xC0:
             LED_RT_OFF();
-            TWCR |= (1<<TWINT) | (1<<TWEA);
+            control |= (1<<TWEA);
             break;
 
-        /* illegal state -> reset hardware */
-        case 0xF8:
-            TWCR |= (1<<TWINT) | (1<<TWSTO) | (1<<TWEA);
+        /* illegal state(s) -> reset hardware */
+        default:
+            control |= (1<<TWSTO);
             break;
     }
+
+    TWCR = (1<<TWINT) | control;
 } /* TWI_vect */