[Open-graphics] Re: Re: DDC injection box Re: Boot-time video mode

[mykrowatt at comcast.net]

 -------------- Original message ----------------------
From: Jon Smirl <jonsmirl at gmail.com>
> 
> Decoding I2C is normally done by software on the host. For a radeon there
> is one io port/memory location that represents the clock and data of the
> I2C stream. Software sits in a loop reading the data and decoding the I2C
> protocol. Check out the Linux I2C drivers. AFAIK all you have to do is get
> the I2C signal off the DVI connector and onto a visible IO address.

   Hmmm, I think I2C is timing-sensitive.  That must tie up the CPU full-time while it's uploading the data.  But it's only for a few milliseconds during boot, so I suppose that's OK.

 
> Note that there are two types of DDC, DDC1 and DDC2. DDC2 has replaced
> DDC1 but there are monitors out there running DDC1. DDC1 and DDC2 are
> different electrically.

   Yeah, the documents the URLs pointed to earlier said that.  Well, if the BIOS implements the low-level protocol by bit-twiddling, it should be able to handle more than one timing format.

   So, it's only the external tool that would need an embedded microcontroller to perform I2C, when impersonating a monitor's EDID behavior.