You can use hardware for one, and software for the other - take your pick which is which.

If you mean connect both I2C and SPI devices to the same MSSP port and have the PIC switch back and forth between I2C and SPI modes to communicate with only one set of devices on the line, then yes. Obviously, you can't send an SPI and I2C signal over the same set of wires at the same time.

You write code that reconfigures the port back and forth between the I2C and SPI functionality when switching modes. You also need to take care that I2C and SPI messages don't get read by the wrong device. Preventing SPI devices from reading I2C messages is easy since you jsut disable all SPI devices with chip select when sending I2C messages.

Stopping I2C devices from reading SPI messages is a bit trickier. Before enabling SPI devices to send them a message, you send an I2C stop-bit to tell all I2C devices that the I2C message is over, and to stop listening to the line until they hear a start bit (this is standard when sending I2C so the device knows the message is actually over). And while you transmit SPI messages, you have to watch the two SPI wire being shared with the I2C so that they do not accidentally imitate an I2C start/stop bit which will wake up the I2C devices and cause them to start listening in (or even worse, if the SPI message happens to imitate the right address and read/write bits, it might initiate a read sequence where the I2C device sends out data which would make for a very difficult and random bug to hunt down).

http://www.best-microcontroller-proj...-star-stop.png

In I2C, all SDA logic transitions only occur while SCL is low except for the stop/start bits. This means you must make sure that the SDA wire only transitions while SCL is low while sending SPI messages. Any SDA transition while SCL is high will be interpreted as a either a start or stop bit by the I2C devices.

Thanks for the detailed responses....I don't think I can actually disable the 12C device (real time clock), so I'll try finding a pic with 2 ports. Ill go with a hardware interface if a pic like this is not readily available.

Originally Posted by fromansr

Thanks for the detailed responses....I don't think I can actually disable the 12C device (real time clock), so I'll try finding a pic with 2 ports. Ill go with a hardware interface if a pic like this is not readily available.

Why bother?, it's pretty easy to do I2C master in software, and there's an example of reading a clock chip in my tutorials.

You aren't disabling the I2C device. It still runs doing whatever it does. You are just enabling or disabling whether or not it is paying attention to what is on the bus. Chip select for SPI also doesn't disable the device- it just tells the SPI device whether or not to pay attention to what is being sent on the bus.

I assume yuo don't mean just branching one pin from the PIC out to be a bunch of enable/disable pins so that they all go on or off?

With a decoder, you can get n pins on the PIC to activate (or deactivate) one pin out of 2^n pins and deactivate (or activate) the rest. THese in turn can be branched out like I described above so n pins can control the state of 2^n groups of pins.