Call Wire.endTransmission but keep the connection active with Wire.endTransmission(false);?

[rwaldron]

https://github.com/firmata/arduino/blob/master/examples/StandardFirmata/StandardFirmata.ino#L101

Working on reading a MMA8452Q (triple axis accelerometer), ran the sketch here and it works fine, but no dice over Firmata. Changing this line to Wire.endTransmission(false); (as seen in the sketch) makes it work correctly.

I'm wondering what, if any, implications exist by doing this?

[soundanalogous]

Have you tested with that change against other I2C devices?

[soundanalogous]

Looking into Wire.endTransmission I think we'll need to add an additional configuration parameter for I2C in Firmata. By default the value will be true, but you'd be able to configure as false. Could be added to I2C_CONFIG to add this globally or could use the currently reserved bit 6 of byte 4 of I2C_REQUEST. Adding to I2C_CONFIG is probably the better approach. I should be able to get this in for Firmata 2.4.

[rwaldron]

Have you tested with that change against other I2C devices?

Unfortunately I had company for the rest of the weekend and couldn't do as much device testing as I would've liked, so no I did not (not enough to count).

(snip)...I should be able to get this in for Firmata 2.4.

That works for me, let me know when you're ready for testing and I will line up the I2C devices I have and run some tests. I will be away starting tomorrow morning through next Sunday.

[soundanalogous]

There's another thing to try here. Typically with I2C when initiating a multi-byte read you can OR the register address with 0x80 to set auto increment the register pointer (which should theoretically achieve the same as endTransmission(false)). Perhaps the MMA8452Q doesn't implement this part of the I2C protocol, but it's worth a try (I don't have this particular sensor so I can't test it myself).

In MMA8452Q_BasicExample change lines 157 and 158 to the following:

Wire.write(addressToRead | 0x80);
Wire.endTransmission();

I'm curious to see if this works. It's a non-standard i2c feature but most vendors implement it.

[tcr]

In a library for the temp/humidity sensor Si7005, the same issue occurs. Hardcoding Wire.endTransmission(false) does allow I2C to work through Firmata.

I tried the approach of ORing with 0x80 in the library https://github.com/jjalling/Arduino-Si7005/blob/master/Si7005.cpp#L30 and had no luck, unfortunately. I can try other suggestions for this module if you'd want.

[soundanalogous]

It looks like I'll have to update the Firmata I2C protocol to include an option to enable auto restart. What I need to test however is if you have more than one I2C device on the bus and one needs auto restart and one doesn't, how to handle that use case.

[soundanalogous]

I've added a fix for this issue. Please try the i2c-restart branch with one or more i2c devices that need the auto restart (MMA8452Q) and one or more devices that use the default (stop) behavior (ADXL345, HMMC5883, etc). I have tested in the default (stop) mode with an HMC6352 digital compass without issue but I don't have any devices that require the restart behavior so I'm unable to test that.

The protocol for i2c read/write updated as follows:
I2C read/write request

0  START_SYSEX (0xF0)
1  I2C_REQUEST (0x76)
2  slave address (LSB)
3  slave address (MSB) + read/write and address mode bits
   {7: always 0} + {6: restartTX, 0 => false (default), 1 => true} + {5: address mode, 1 => 10-bit mode} +
   {4-3: read/write, 00 => write, 01 => read once, 10 => read continuously, 11 => stop reading} +
   {2-0: slave address MSB in 10-bit mode, not used in 7-bit mode}
4  data 0 (LSB)
5  data 0 (MSB)
6  data 1 (LSB)
7  data 1 (MSB)
...
n  END_SYSEX (0xF7)

The change is in bit 6 of byte 3 (this bit was formerly unused). Set this bit to 1 for devices that need the auto restart functionality. The default value should be 0 (which it most likely is in any Firmata client library that supports i2c). I had to invert the logic here in order to make sure backwards compatibility would be maintained.