The original ubb-vga used fancy semiconductors (diodes) to generate the 0.7 V analog color signals. I've now made a simpler version with only resistors:
http://downloads.qi-hardware.com/people/werner/ubb/vga/ubb-vga2-prototype.jpg This looks a bit more chaotic that it really has to be, because I've also been experimenting with some other signals and there are still some remains of those experiments in evidence, e.g., the 1 kOhm resistor pointing straight up. David Kuehling has suggested the use of the Ben's MMC controller to generate the pixel data. One problem with MMC/SD/SDIO is that these protocols require handshakes with the device. For example, a block write looks like this: 1. host sends the command on CMD 2. device sends a reponse on CMD 3. host sends the block on DAT0-DAT3 4. device sends a handshake on DAT0 5. if sending multiple blocks, repeat steps 3 and 4 For further details, see http://www.jedec.org/sites/default/files/docs/JESD84-A441.pdf (requires registration but doesn't need a valid e-mail address) The response has a start bit (0) followed by some data, and then a stop bit (1). The handshake is pulling DAT0 low while busy. Since we don't have a real MMC/SD/SDIO device, we need to fake these responses. I'll leave faking the DAT0 handshake for later. In any case, we need to fake the command response, or the transfer will never start. I found a simple way to do this: add a 330 Ohm pull-down to CMD, use CMD for HSync, and switch CMD briefly from GPIO to the MMC controller so that it can see the "response". Luckily, it doesn't try to interpret the response code. By aligning the response's start bit with (active-low) HSync pulse, the whole kludgery is transparent to the VGA monitor. Here is a first impression of what this looks like: http://downloads.qi-hardware.com/people/werner/ubb/vga/ubb-vga2-first.jpg The timing is more accurately VGA than what I had before. Note that the whole display content now fits on the screen and all pixels have the same width. The timing also happens to be a bit more fragile in the sense that the monitor sometimes gets very confused. Not sure yet what causes this. - Werner _______________________________________________ Qi Hardware Discussion List Mail to list (members only): [email protected] Subscribe or Unsubscribe: http://lists.en.qi-hardware.com/mailman/listinfo/discussion
