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May 31, 2008
about 8 months ago
The “shift” I mention is just something you have to do when you “assemble” the control byte for I2C commands. What was confusing to me is that for this display all the examples were done using the Arduino “wire” library and show commands like this:
which is easy to use but if you are trying to understand what’s really going on it’s not very helpful.
The control byte is part of the I2C spec and if you are interested in learning more about I2C communication here is a link to a page that does a pretty good job of providing details on how I2C works. It is definitely more challenging than SPI but many MCUs these days have built in peripherals to handle both.
Finally figured it out. I dragged out my Arduino board and used their sample code and then hooked up my Saleae logic analyzer (worth it’s weight in gold, btw) to see what they were actually sending. The control byte does use 0x71 as the device address but it gets shifted one bit to the left and then the R/W bit (set low) is added in. So you end up sending 0xE2 for the control byte. So for example if you want to display the numbers 5,6,7 and 8 on the display you would send this sequence: Start - 0xE2 - 0x05 - 0x06 - 0x07 - 0x08 - Stop. If they had just shown something like this in the documentation it would have been really really helpful. I also discovered that apparently the display unit has pullup resistors for the SCL and SDA lines so you don’t have to add those yourself. I don’t know what the max clock rate is for the display but I was using 100Khz.
I was afraid of that - but on the bright side, at least I don’t feel like an idiot any more. It shouldn’t be this hard. Hey Sparkfun - has ANYONE out there been able to get this display to work using I2C with something other than an Arduino??? If so please let us in on the secret.
Thanks Mark - I am anxious to hear what you find when you receive this version of the display. I tried sending exactly what you mention and never got the display to acknowledge the control byte (0x71 plus the R/W bit). I also had pullup resistors on both SCL and SDA. I tried numerous variations on the control byte and never got it to do anything. On the bright side, when I gave up and switched to using the SPI port it worked the first time.
At least for the I2C mode, this is the worst documentation I have ever seen. There is no mention anywhere of max clock speed, if pull-up resistors are needed or even a sample command using I2C. Has anyone got this to work and can at least explain how the device address byte with the standard R/W bit is sent to the display?
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