Weird limit on data size when writing to I2C

[reagle 7]

Stumbled on this strange behavior when using Energia with 430G2553 Launchpad and also Fraunchpad. Any time I try to do a long write to I2C device, only 17 bytes get put on the actual wire.

What am I missing here?

 

Code typically looks like this- I start transmission, sit in the loop sending data and then end transmission.

Here is an example with a bunch of debug :

 

{

    byte temp=0;

    Serial.println("Transmitting display data");

    Wire.beginTransmission(SSD_ADDR); 

    Serial.print("\t");

    Serial.print(SSD_DUMMY_BYTE_C,HEX);

    Serial.print("\t");

    Serial.println(Wire.write(SSD_DUMMY_BYTE_C));

 

    Serial.print("\t");

    Serial.print(0xA2,HEX);

    Serial.print("\t");

    Serial.println(Wire.write(0xA2));

    

    Serial.print("\t");

    Serial.print(SSD_DUMMY_BYTE_D,HEX);

    Serial.print("\t");

    Serial.println(Wire.write(SSD_DUMMY_BYTE_D)); 

    while(len--)

    {

         Serial.print("\t");

         Serial.print(len);

         Serial.print("\t");

         temp=*databuf++;

         Serial.print(temp,HEX);

         Serial.print("\t");

         Serial.println(Wire.write(temp));        

    }    

    Serial.println(Wire.endTransmission());    

}

 

 

Len is usually 30-60 bytes, and databuffer is sized appropriately. I see the code iterate through it correct number of times, but less bytes are sent out.

This is in Energia 14 at the moment, though I tried newer version as well. 

Typical output is below. You can see how write status goes from 1 to 0 after 17 bytes. The last value is return status from Wire.endTransmission, and it is 0, so it appears the system thinks all is happy

 

 

 

Transmitting display data

80 1

A2 1

0 1

67 40 1

66 55 1

65 55 1

64 1 1

63 0 1

62 0 1

61 0 1

60 54 1

59 55 1

58 15 1

57 0 1

56 0 1

55 0 1

54 40 0

53 55 0

52 55 0

51 1 0

50 0 0

49 0 0

48 0 0

47 0 0

46 55 0

45 50 0

44 55 0

43 0 0

42 0 0

41 0 0

40 A0 0

39 A0 0

38 AA 0

37 A 0

36 0 0

35 0 0

34 0 0

33 0 0

32 54 0

31 55 0

30 15 0

29 0 0

28 0 0

27 0 0

26 80 0

25 AA 0

24 AA 0

23 2 0

22 0 0

21 0 0

20 0 0

19 50 0

18 55 0

17 1 0

16 54 0

15 0 0

14 0 0

13 0 0

12 A0 0

11 A0 0

10 AA 0

9 A 0

8 0 0

7 0 0

6 0 0

5 40 0

4 55 0

3 55 0

2 5 0

1 0 0

0 2 0

0

 

 

 

[reagle 7]

Also tried using Serial.println(Wire.write(databuf,len));

Status comes back with the right number of bytes sent, but once again I only see 17 of them on the bus before the Stop condition  is issued 

[Rei Vilo 695]

Check the size of the buffer for I2C on the Wire library. It may be limited to 16 bytes.

[solipso 8]

Even Pete Scargill got bitten by this. See http://tech.scargill.net/i2c-the-easy-way/

[reagle 7]

Oh, that makes sense!

Buffer is indeed 16 bytes, but it appears to "work" if I issue repeated start after the first command bytes go out.

Basically if I add   Serial.println(Wire.endTransmission(false)); before the big loop, then it does send the right number of bytes.

Now the bytes themselves are not what they should be so I am probably just crawling some random memory location

Will fiddle with buffer size instead. Thanks!