Do not rely on Clockstretching for writes

README.md

@@ -10,10 +10,10 @@ Currently the following AVR MCUs are supported:
 
 AVR MCU | Flash bytes used (.text + .data) | Bootloader region size
 --- | --- | ---
-atmega8 | 718 (0x2CE) | 512 words
+atmega8 | 802 (0x322) | 512 words
-atmega88 | 740 (0x2E4) | 512 words
+atmega88 | 826 (0x33A) | 512 words
-atmega168 | 740 (0x2E4) | 512 words
+atmega168 | 826 (0x33A) | 512 words
-atmega328p | 740 (0x2E4) | 512 words
+atmega328p | 826 (0x33A) | 512 words
 
 (Compiled on Ubuntu 18.04 LTS (gcc 5.4.0 / avr-libc 2.0.0) with EEPROM and LED support)
 
@@ -74,7 +74,7 @@ Read chip info | **SLA+W**, 0x02, 0x00, 0x00, 0x00, **SLA+R**, {8 bytes}, **STO*
 Read 1+ flash bytes | **SLA+W**, 0x02, 0x01, addrh, addrl, **SLA+R**, {* bytes}, **STO** |
 Read 1+ eeprom bytes | **SLA+W**, 0x02, 0x02, addrh, addrl, **SLA+R**, {* bytes}, **STO** |
 Write one flash page | **SLA+W**, 0x02, 0x01, addrh, addrl, {* bytes}, **STO** | page size as indicated in chip info
-Write 1+ eeprom bytes | **SLA+W**, 0x02, 0x02, addrh, addrl, {* bytes}, **STO** |
+Write 1+ eeprom bytes | **SLA+W**, 0x02, 0x02, addrh, addrl, {* bytes}, **STO** | write 0 < n < page size bytes at once
 
 **SLA+R** means Start Condition, Slave Address, Read Access
 
@@ -82,16 +82,28 @@ Write 1+ eeprom bytes | **SLA+W**, 0x02, 0x02, addrh, addrl, {* bytes}, **STO**
 
 **STO** means Stop Condition
 
+A flash page / eeprom write is only triggered after the Stop Condition.
+During the write process twiboot will NOT acknowledge its slave address.
+
 The multiboot_tool repository contains a simple linux application that uses
 this protocol to access the bootloader over linux i2c device.
 
 The ispprog programming adapter can also be used as a avr910/butterfly to twiboot protocol bridge.
 
+
+## TWI/I2C Clockstretching ##
+While a write is in progress twiboot will not respond on the TWI/I2C bus and the
+TWI/I2C master needs to retry/poll the slave address until the write has completed.
+
+As a compile time option (USE_CLOCKSTRETCH) the previous behavior of twiboot can be restored:
+TWI/I2C Clockstretching is then used to inform the master of the duration of the write.
+Please note that there are some TWI/I2C masters that do not support clockstretching.
+
+
 ## Development ##
 Issue reports, feature requests, patches or simply success stories are much appreciated.
 
 
 ## Roadmap ##
 Some ideas that I want to investigate / implement in twiboot:
-- find a way to not rely on TWI clock stretching during write access
 - support AVR TINYs (USI peripheral, no bootloader fuse, no Read-While-Write flash)

main.c

@@ -24,6 +24,7 @@
 #define VERSION_STRING      "TWIBOOT v2.1"
 #define EEPROM_SUPPORT      1
 #define LED_SUPPORT         1
+#define USE_CLOCKSTRETCH    0
 
 #define F_CPU               8000000ULL
 #define TIMER_DIVISOR       1024
@@ -63,6 +64,8 @@
 #define CMD_ACCESS_CHIPINFO     (0x10 | CMD_ACCESS_MEMORY)
 #define CMD_ACCESS_FLASH        (0x20 | CMD_ACCESS_MEMORY)
 #define CMD_ACCESS_EEPROM       (0x30 | CMD_ACCESS_MEMORY)
+#define CMD_WRITE_FLASH_PAGE    (0x40 | CMD_ACCESS_MEMORY)
+#define CMD_WRITE_EEPROM_PAGE   (0x50 | CMD_ACCESS_MEMORY)
 
 /* SLA+W */
 #define CMD_SWITCH_APPLICATION  CMD_READ_VERSION
@@ -198,6 +201,22 @@ static void write_eeprom_byte(uint8_t val)
 
     eeprom_busy_wait();
 } /* write_eeprom_byte */
+
+
+#if (USE_CLOCKSTRETCH == 0)
+/* *************************************************************************
+ * write_eeprom_buffer
+ * ************************************************************************* */
+static void write_eeprom_buffer(uint8_t size)
+{
+    uint8_t *p = buf;
+
+    while (size--)
+    {
+        write_eeprom_byte(*p++);
+    }
+} /* write_eeprom_buffer */
+#endif /* (USE_CLOCKSTRETCH == 0) */
 #endif /* EEPROM_SUPPORT */
 
 
@@ -279,6 +298,19 @@ static uint8_t TWI_data_write(uint8_t bcnt, uint8_t data)
         default:
             switch (cmd)
             {
+#if (EEPROM_SUPPORT)
+#if (USE_CLOCKSTRETCH)
+                case CMD_ACCESS_EEPROM:
+                    write_eeprom_byte(data);
+                    break;
+#else
+                case CMD_ACCESS_EEPROM:
+                    cmd = CMD_WRITE_EEPROM_PAGE;
+                    /* fall through */
+
+                case CMD_WRITE_EEPROM_PAGE:
+#endif /* (USE_CLOCKSTRETCH) */
+#endif /* (EEPROM_SUPPORT) */
                 case CMD_ACCESS_FLASH:
                 {
                     uint8_t pos = bcnt -4;
@@ -289,19 +321,19 @@ static uint8_t TWI_data_write(uint8_t bcnt, uint8_t data)
                         ack = 0x00;
                     }
 
-                    if (pos >= (sizeof(buf) -1))
+                    if ((cmd == CMD_ACCESS_FLASH) &&
+                        (pos >= (sizeof(buf) -1))
+                       )
                     {
+#if (USE_CLOCKSTRETCH)
                         write_flash_page();
+#else
+                        cmd = CMD_WRITE_FLASH_PAGE;
+#endif
                     }
                     break;
                 }
 
-#if (EEPROM_SUPPORT)
-                case CMD_ACCESS_EEPROM:
-                    write_eeprom_byte(data);
-                    break;
-#endif /* (EEPROM_SUPPORT) */
-
                 default:
                     ack = 0x00;
                     break;
@@ -392,6 +424,30 @@ static void TWI_vect(void)
 
         /* STOP or repeated START -> IDLE */
         case 0xA0:
+#if (USE_CLOCKSTRETCH == 0)
+            if ((cmd == CMD_WRITE_FLASH_PAGE)
+#if (EEPROM_SUPPORT)
+                || (cmd == CMD_WRITE_EEPROM_PAGE)
+#endif
+               )
+            {
+                /* disable ACK for now, re-enable after page write */
+                control &= ~(1<<TWEA);
+                TWCR = (1<<TWINT) | control;
+
+#if (EEPROM_SUPPORT)
+                if (cmd == CMD_WRITE_EEPROM_PAGE)
+                {
+                    write_eeprom_buffer(bcnt -4);
+                }
+                else
+#endif /* (EEPROM_SUPPORT) */
+                {
+                    write_flash_page();
+                }
+            }
+#endif /* (USE_CLOCKSTRETCH) */
+
             bcnt = 0;
             /* fall through */