Bad Example code Ex_Slave.c or what ?

soonc · Joined: 03 Dec 2013 Posts: 215

I'm having problems understanding the Ex_Slave.c example given by CCS.

It's copyrighted so I can't publish all of it here, but I'll reference portions to clarify my questions.

The ssp_interrupt() uses a global variable named "address".

address is not initialize in main() or anywhere else as far as I can see.

There is also a global array named buffer[16].

In the ssp_interrupt() code

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

At the start of ex_slave.c it says this:

Ttelmah · Joined: 11 Mar 2010 Posts: 19515

Several of the examples are 'weak'. In ex_sisr for example, it is not explained that with the code as supplied, the buffer size must be 'binary' (2,4,8,16 etc.). There are lots of similar "oh dear" problems with examples, but most are enough to point in the right direction.

In this case, address is initialised from the I2C packet. The code does not error check, so if a wrong address is sent, 'splat'. The packet has to be laid out as if it were for a 2401 or 2402 EEPROM. The reason for the small size supported, is that the code was written to be useable on even the smallest chips. Given how little RAM some have, a 16byte limit makes sense.

There is always a 'confusing complexity' in I2C, between the 'device address', and the 'register address'. The standard I2C packet comprises:
Start
Device address (W)
Register address
Then for a write, the bytes to write
Or for a read, 'restart', device address (R), and bytes then read.
Followed by a stop.

The master should NACK the last byte being read on a read transaction.

This format is described in the I2C documentation, and reading this must be a starting point.

However key thing is that you must read the header of the example. As PCM_programmer points out, this tells you what driver to use for the master, and the address limitations.

Used as a pair with the 2401 driver, the code works. If you send it an invalid register address, then it fails.

soonc · Joined: 03 Dec 2013 Posts: 215

Thanks for the explanation.
I got it all to work by adding my own code to do what was lacking without using the "other" files.

I'd prefer if an example has a main() then it should be complete. Reading example code is frustrating enough without switching from file to file.
All on one page makes more sense to me.

Anyway: PCM programmer's I2C scanner did the trick again...

Thanks everyone.

Happy New Year.