i2cspi
We have a product which uses PIC18F4520, and we used some devices like LCD driver and so on, that all use I2C bus to comunicate with the micro. But now we want to add a couple of AD5504, which is a DAC with a SPI interface. We now have a question as to whether they could share the same connection with the I2C bus or not.
Do you have a good suggestion about it?
Some folks are having a hard time visualizing things connected together, so here's a picture:
(Serving suggestion)
simulate this circuit – Schematic created using CircuitLab I2C is a bus, so like-named signals are connected together. The addressing scheme allows the microcontroller to select which device it's talking to. On the Arduino, the 10kΩ pull-up resistors go to VDD (Usually +5V or +3.3V).
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:
Best Answer
Another option would be to use an SPI to I2C bridge chip. This would essentially allow you to add a chip select for your entire I2C bus. One such option is shown below.
http://www.silabs.com/products/interface/spitoi2c/Pages/default.aspx