A couple of questions

The Captain

I'm making a little project and have been pondering of these for a while.

1. Reading the Full Duplex object, just for clarification, it only requires 2 pins?
2. Is it possible to create an 8 bit R-2R Ladder DAC generate red, blue and green for VGA?

Mike Green

1) Yes. The FullDuplexSerial object uses one pin for transmit data and the other pin for receive data. It's possible to use only one pin for half duplex communications.
2) The VGA setup uses 3 x 2-bit R-2R Ladder DACs, one for each of red, blue, and green. There are also protective resistors in the outputs for H-sync and V-sync. Look at the schematic for the Propeller Protoboard.

deSilva

(ad 2) you can easily make a 6 bit grayscala VGA signal using an R-2R network
doing more in color is possible but needs more than one COG (see: "Baggers"), Video generation for VGA is relatively simple.

Ken Peterson

@Captain: If you were to do what deSilva suggests, x 3, then you can have 6 to 8 bits for each color. You'll still need a couple of bits for your vertical and horizontal sync signals. You may run into memory issues with all that color depth, however.

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The more I know, the more I know I don't know.· Is this what they call Wisdom?

IAN STROME

Hi all,
Do you need something like attached jpg.
The resistors are for 1V P to P o/p, Not sure whether it should be
0.7V into 75 ohm ? DeSilva knows I should think!
In which case resistors should be approx. 4K0,2K0,1K0,500R
The 2 select lines effectively give you 4 palettes of 64 colours.
Yes Ken with 8 bit VGA memory probs, but can't have everything.

Best To All
Ian

:- A minor operation is one performed on somebody else!

deSilva

The tricky thing will be the synchronisation, as the SELECT pins can be set too early
ex:

ANDN OUTA, selectmask
OR OUTA, selectbits 
WAITVID

The exact value ofthe resistors is not too important; you will hardly notice deviations around +/- 20% I NEVER use anything else but E3 values 1 - 2.2 - 4.7 (even for LEDs: either you need it bright: then take 220 , or if not so: take 470 ...)

But the DAC is also tricky! Note that you have a lot of inter-colour connections through the select nodes.
The video DAC is not a R-2R ladder, which needs high impedance buffering. It is rather a CURRENT ADDER, letting the sum of many current sources flow into 75 Ohms....

Ken Peterson

just to add to that, the 240 ohm resistors in the sync lines are not just for protection. If using a high impedance input for TTL, you will get 3.3V, but if you connect it to a 75ohm load, you will get approximately 0.7V. I am working with a different device that does RGB with Comp Sync and it is very similar.

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The more I know, the more I know I don't know.· Is this what they call Wisdom?

deSilva

Obviously...Who said? Ah, Mike