I2C pin config and signal aspects in the ports

mvaraujo · Joined: 20 Feb 2004 Posts: 59 Location: Brazil

Hi!

I have some problems with i2c master mode comm using a PIC18F452@40MHz and would like to take your thoughts about that.

How do you config the pin for hardware i2c? Output or input?

Datasheet from uChip says both input. In my design, if I set them like these I get a strange CLK signal (slow ramp up of the voltage from 0 to 5V) with 90KHz. If set them to output, the CLK signal looks great, 100khz perfect as expected.

Thanks

Marcus

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

Look in the CCS Help area. It talks about set_tris_c(). You can set the SDA and SCL pins to inputs using this command.

Ronald

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

I tested your problem on my demo board, and it worked OK.
I didn't test this with the 4x PLL, because my demo board
is not setup for that. So I ran it at 4 MHz.

The following program produced i2c clock and data signals
with fast rise and fall times. At the start of the program,
I set pins C3 and C4 as inputs.

mvaraujo · Joined: 20 Feb 2004 Posts: 59 Location: Brazil

PCM Programmer,

And which are the pull-up resistors of your demo board on the i2c lines?

It's even more strange because if I use 10k pull-ups, I have slow ramp-ups, if you use 1k, SDA is fast and clear but a 0.5V positive offset appears on the SCL. Crazy, huh?

I'll figure it out, sooner or later!

Thanks,

Marcus

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

1K pullups are out of spec for the i2c bus. For a 5v i2c bus,
the smallest pullup size is 1.6K. So I suggest you use 2.2K
on your demo board. That's a convenient size that you
probably have in your parts cabinet.

mvaraujo · Joined: 20 Feb 2004 Posts: 59 Location: Brazil

I understand your point. In fact it's not a demo board, it's power monitoring device that I'm design. I'll try out the 2k2 to how it works.

Thanks,

Marcus