arduinoi2cspi
I know mixing SPI and I2C is a bad idea, but, when using a microcontroller short on pins, is it safe to share clocks between both buses, provided both are running at bus-tolerable frequencies?
In my application, I plan on using one bus at a time, but I suppose this question applies even if that was not the case.
I've seen exactly the behavior you're afraid of on an ADXL345, which uses the same I2C/SPI selection scheme. I had another SPI device that used different clock polarity and it happened to emulate an I2C start code, the ADXL345 tried to talk out of turn as I2C. Bad news.
I carefully rewrote the SPI as bit bang instead of using the peripheral, making sure to not change the MOSI line while the clock was high. (This is the I2C start condition.) That seemed to solve things.
If I was starting from scratch, I would try to use I2C bus instead or a dedicated SPI port for the ADXL345.
Apparently I wan't the only one to encounter this. This paragraph appeared in a later revision of the ADXL345 datasheet:
The choice really makes little difference when you are using the Arduino libraries. You should see the same type of performance.
As for the i2c pullup resistors, the BMP085 board you link to has pullups already on the board, connected to a closed solder jumper. Those same pullups should be enough for both sensors, so you wouldn't need to add more.
But there is no reason the sensor shouldn't work on SPI with a sd card as well. Did you double check that you tied all grounds together? How does it not work? Does it only work for one read/write before failing? Are you making sure you drive both CS pins to a high/low, instead of leaving one as a floating input?
Best Answer
You could theoretically do this but you'd have to use two different buffers. The system I'm envisioning would have a bidirectional buffer with tri states might work (or maybe two) and you could switch the SPI bus on and the I2C bus off.
The pull ups for I2C could be on the other side of the buffer
However if one is using a small micro, they are probably doing so to save on space and you might as well use a bigger micro than a micro and a buffer (or two).