i2c_read() on PIC 12F629

elellilrah · Joined: 29 Aug 2007 Posts: 14

Hello,

I am looking at using a PIC12F629 as a slave to do some very simple operations using the I2C bus for a display. However, the help in PCW 4.082 indicates that i2c_read() only works on chips with dedicated hardware. Is true? I've seen code snippits of I2C done with software mode on any many different processors and pins. I just want to verify that the i2c_read function will work.

Thanks!

Ttelmah · Guest

They won't.
It is possible to write an I2C slave, without the hardware, but the CCS functions don't support this, and it really is processor intensive. You would not be able to do anything else on a 629, since the slave _must_ react in very tight timing windows, and even simple display functions would without hardware buffering be very hard to guarantee to finish in time.
I'm afraid that you would be much better off selecting a basic chip with I2C hardware.
I'd put the labour cost to do this reasonably well, to probably be in excess of the cost of 1000 'better' processors...

Best Wishes

elellilrah · Joined: 29 Aug 2007 Posts: 14

crystal_lattice · Joined: 13 Jun 2006 Posts: 164

In your first post you said:

Ttelmah · Guest

Spot on.
The key to understand, is that the _master_, controls the timings on the I2C bus. If it runs slow, or has to do something else, it doesn't matter. The bus just runs slow. However the slave, _must_ respond inside the timings being generated by the master (there is an ability to 'delay' the end of the response (clock stretching), but this already requires the slave to be responding, and have held the clock line low.
You can implement an I2C master on the 12F629, as easily as anything. A _slave_, is the hard bit.
Talking to an EEPROM as a master, with the 629, is 'easy'. There are probably several hundred examples here on chips similar to this, that will work fine. Receiving I2C as a slave, and writing it to a display, is a whole different 'can of worms'...
If (for instance), the bus is running at 100KHz, the slave device, has just 10uSec, in which it must either have read the data bit, or have held the clock line low.

Best Wishes

crystal_lattice · Joined: 13 Jun 2006 Posts: 164

If you have to use the 629, you could buffer the incomming data and only after the complete receiption of a message, do the display function. The 629 can then have a ready flag which the master will have to poll before starting a transmission.

crystal_lattice · Joined: 13 Jun 2006 Posts: 164

I see Ttelmah has already mentioned buffering... Shocked

elellilrah · Joined: 29 Aug 2007 Posts: 14

zcashion · Guest

If you plan on using a third line for a chip select, then you should probably switch to SPI communication. Using a chip select pin circumvents the need for sending the address byte that is necessary with I2C.

As for your question on I2C_read. This command is dependent upon whether you are operating in Master or Slave mode. In Master mode it generates a clock. In Slave mode it waits indefinitely for a clock from the master. This is why there is an option to restart the watchdog timer in the #use i2c command.

Cheers

rnielsen · Joined: 23 Sep 2003 Posts: 852 Location: Utah

Simply put, if you want to use a PIC as a slave then select one that has the hardware to support it. Otherwise, you're in for a very bad headache and loss of hair.

Ronald