I2C Baud Rate Calculation

tmuir · Joined: 04 Apr 2010 Posts: 3

I'm doing 400kHz I2C on a dsPIC30F4011. I've got everything working, but I still dont understand the baud rate calculation. I realize that CCS takes care of that for you, but I'm trying to understand exactly whats going on in every register.

According to the dspic30F family reference manual: http://ww1.microchip.com/downloads/en/DeviceDoc/70046D.pdf page 550, The equation for I2CBRG is (Fcy/Fscl - Fcy/1,111,111) - 1. I'm using a 7.3728 MHz internal oscillator with a 8X PLL, making Fcy = 14.7456MHz. If Fscl is 400kHz, I2CBRG ends up being 22. Using this value, the I2C does not work.

Looking at the .lst file generated by CCS, using the same values above for Fcy and Fscl, I2CBRG is set to 0x5D, or 93. Using this value, I2C works.

Is the FRM wrong? What is the right calculation?

Ttelmah · Joined: 11 Mar 2010 Posts: 19368

What CCS setup line are you using for the I2C?.
What compiler version?.
Post your fuse line, clock line, and I2C setup line.
Are you _sure_ all your I2C hardware does work OK at 400K (pull up's low enough, bus capacitance low enough, etc. etc..).

I'd say that CCS is probably getting the calculation wrong, but your hardware is not successfully running at 400K. It just 'happens' that the wrong value from CCS, brings the bus rate low enough to make it work....

Best Wishes

tmuir · Joined: 04 Apr 2010 Posts: 3

PCWHD v. 4.079

Ttelmah · Joined: 11 Mar 2010 Posts: 19368

As far as I know, the nunchuck, uses I2C, at 100KHz, not 400KHz....

Best Wishes

Ttelmah · Joined: 11 Mar 2010 Posts: 19368

Update on that.
Have just done a search, and you will find in the Nunchuck forums, people saying it does run at 400KHz, _but_ you have to add delays between the transmitted bytes, in the order of 8uSec, before it works reliably at this rate. Apparently the wireless units, have to use 400K (won't work at 100K), but the timings between the bytes, need to stay at the 100K spacing, or it won't work...
A search on Google, will find a _lot_ about this.

Best Wishes

tmuir · Joined: 04 Apr 2010 Posts: 3

Everything I had read said 400kHz, but I'll believe my oscilloscope over the internet any day of the week. You're right, there is a ton of wii nunchuck info on google, mostly for arduinos. The best site I found breaks down just about every wii peripheral's data format: http://wiibrew.org/wiki/Wiimote/Extension_Controllers#Data_Format_.28Classic_Controller.29

For future reference though, if I truly need 400kHz I2C, where in my code do I need to set I2CBRG to the correct value, to make sure that it is not overwritten by everything that comes along with #use_i2c()?

Ttelmah · Joined: 11 Mar 2010 Posts: 19368

First, I'd report the baud rate error to CCS.
With luck, they should give you an update for this, and that way you can go back to using the standard functions. Smile

At the start of the main. Things like baud rate setups, are done just once, in the 'pre-header', that the compiler generates just above the main. Only when you have things that change settings (like 'set_uart_speed' on the normal serial), are changes made 'later'.

Best Wishes