When you reset a Uno running the Optiboot loader, the bootloader first flashes pin 13 three times.
Top line (gray) is sent to the Arduino, middle line (orange) is sent from the Arduino.
While that is happening, the program avrdude running on your computer is sending a query to the device:
STK_GET_SYNC / CRC_EOP (0x30/0x20) The Arduino doesn't notice the first "get sync" because it is busy flashing pin 13. Once it is done it notices the "get sync" (it would be buffered by the serial hardware) and replies:
STK_INSYNC / STK_OK (0x14/0x10) It looks like avrdude got a bit impatient, and timed out, because it tries again with the "get sync" query. This time Optiboot responds immediately.
The rest of the uploading is described in the next image. Example produced uploading the stock "Blink" program.
(Click on the image above for a larger version)
The steps are:
Query: Get Sync? Reply: In Sync.
Query: Get parameter? (major version) Reply: version 4.
Query: Get parameter? (minor version) Reply: version 4.
Set device parameters. The following device parameters are sent to the chip:
0x42 // STK_SET_DEVICE 0x86 // device code 0x00 // revision 0x00 // progtype: “0” – Both Parallel/High-voltage and Serial mode 0x01 // parmode: “1” – Full parallel interface 0x01 // polling: “1” – Polling may be used 0x01 // selftimed: “1” – Self timed 0x01 // lockbytes: Number of Lock bytes. 0x03 // fusebytes: Number of Fuse bytes 0xFF // flashpollval1 0xFF // flashpollval2 0xFF // eeprompollval1 0xFF // eeprompollval2 0x00 // pagesizehigh 0x80 // pagesizelow 0x04 // eepromsizehigh 0x00 // eepromsizelow 0x00 // flashsize4 0x00 // flashsize3 0x80 // flashsize2 0x00 // flashsize1 0x20 // Sync_CRC_EOPOptiboot ignores all those and replies with In Sync/OK. :)
Set extended device parameters:
0x45 // STK_SET_DEVICE_EXT 0x05 // commandsize: how many bytes follow 0x04 // eeprompagesize: EEPROM page size in bytes. 0xD7 // signalpagel: 0xC2 // signalbs2: 0x00 // ResetDisable: Defines whether a part has RSTDSBL Fuse 0x20 // Sync_CRC_EOPOptiboot ignores all those as well and replies with In Sync/OK.
Enter program mode. Reply: In Sync/OK.
Read signature. Optiboot replies with
0x1E 0x95 0x0Fwithout actually reading the signature.Write fuses (four times). Optiboot does not write the fuse but just replies In Sync/OK.
Load address (initially 0x0000). The address is in words (ie. a word is two bytes). This sets the address for where the next page of data will be written.
Program page (up to 128 bytes are sent). Optiboot replies "In Sync" immediately. Then there is a pause of about 4 ms while it actually programs the page. Then it replies "OK".
Load address (now 0x0040). This is address 64 in decimal, ie. 128 bytes from the start of program memory.
Another page is written. This sequence continues until all the pages are written.
Load address (back to 0x0000). This is for verifying the write.
Read page (up to 128 bytes are read). This is for verifying. Note that even if the veryify fails, the bad data is still written to the chip.
Leave programming mode.
What does "not in sync" mean?
As you can see from the above, every step through the programming sequence the Arduino is expected to reply with "In Sync" (0x14), possibly followed by some data, followed by "OK" (0x10).
If it is "not in sync" that means that avrdude did not get the "in sync" response. Possible reasons could be:
- Wrong baud rate used
- Wrong serial port selected in IDE
- Wrong board type selected in IDE
- No bootloader installed
- Wrong bootloader installed
- Board not configured to use the bootloader (in the fuses)
- Some device plugged into pins D0 and D1 on the Arduino, interfering with serial commications
- The USB interface chip (ATmega16U2) not working properly
- Wrong clock for the board
- Wrong fuse settings on the Atmega328P (eg. "divide clock by 8")
- Board/chip damaged
- Faulty USB cable (some USB cables provide power only, and are not for data, eg. cheap cables for USB fans)
What is "in sync"?
As mentioned above, the response "In sync" means that the Arduino (bootloader) is synchronised with the uploading program.