SparkFun Forums

[GeneW]

Is someone willing to explain under what Arduino program conditions the "repeated start condition" is sent? I am referring to code using the Wire TWI library.

[theatrus]

It highly depends on the device you are talking to. Repeated starts chain together several I2C messages to one device without having to resend the ID, and can be used as a faster transfer mechanism.

[GeneW]

The VTI SCP1000-D11 pressure sensor is what I am working with currently. It claims to have a TWI interface but during an 8-bit register read, it requires this sequence:

Code: Select all

slave address > write bit > ack > 8 bit slave register address > ack > restart > slave address > read bit > ack > slave register data > nak > 
stop

From the VTI FAQ for the part (not the data sheet as one would expect):

Check the TWI waveforms. Notice that 'Restart' bit can not be replaced by separate "Stop" and "Start" bits (SCP1000 requires the restart bit and it has to be only a Start bit (without a stop bit before it).

Hence my original question.

[GeneW]

Well, after further study of the Wire library, I have concluded that it doesn't support the repeated start condition. It looks like it always inserts a stop condition before repeating the start condition.

If I interpret the I2C specification correctly, this is a deviation from that specification.

Maybe that's why Sparkfun only carries the SPI version of the SCP1000.

[GeneW]

I found the solution here:

http://www.arduino.cc/cgi-bin/yabb2/YaB ... 1242078805

The Wire library doesn't completely implement the I2C standard. This one does:

http://homepage.hispeed.ch/peterfleury/ ... tware.html