267 TT08 VGA FUN!

267 : TT08 VGA FUN!

Design render
  • Author: algofoogle (Anton Maurovic)
  • Description: Rough 24-bit VGA DAC tests with digital control block
  • GitHub repository
  • Clock: 25000000 Hz

Overview

This project (tt08-vga-fun) uses (roughly-designed) current steering DACs to hopefully produce analog outputs that can produce an adequate RGB888 (24-bit) VGA image, based on patterns that can be generated from a simple digital controller. This improves on my previous tt06-grab-bag -- my 1st analog ASIC project, included on TT06, using 3 RDAC instances instead.

With these current steering DACs, I'm hoping for an improved slew rate (estimated to be about 60-80nS; still below the target of 40nS, but better than the TT06 version which was estimated to be about 240nS).

Note that the analog R/G/B outputs (ua[1:3]) are expected to be in the range 0.9-1.8V, and high impedance, while VGA requires a 0.0-0.7V range and 75Ω impedance. Thus, external opamps will be required.

How it works

There is a digital control block which can be controlled by the state of the ui_in pins at reset. It has various test modes, and a pass-through mode.

Here are some of the test patterns it can produce, but note that the image probably won't be this clear because of: (a) poor matching; and (b) slew simulated to be worse than 40nS will lead to a little bit of horizontal smearing:

The digital control block internally drives 3 (RGB) colour channels, each of which has 8 positive and 8 negative polarity bits. This complementary polarity is required for switching the binary-weighted current steering transistors either one way or the other, maintaining an equal (estimated) current of 500µA per channel. Each channel's internal current sum is then converted to a voltage with a pull-up resistor that is about 2.3kΩ.

Additionally the first analog output pin (ua[0]) is the internal VbiasR of the red channel DAC (gate voltage for current mirroring); this is for testing, but could possibly also be pulled up or down a little to see what effect it has on the red channel's output.

How to test

TBC.

External hardware

Probably an op-amp on each analog output, plus a VGA connector.

TBC.

IO

#InputOutputBidirectional
0mode[0] / dac_in[0]r7vblank_out
1mode[1] / dac_in[1]g7hblank_out
2mode[2] / dac_in[2]b7
3mode[3] / dac_in[3]vsync
4mode[4] / dac_in[4]r6
5mode[5] / dac_in[5]g6bias1_in
6mode[6] / dac_in[6]b6bias2_in
7mode[7] / dac_in[7]hsyncbias3_in

Analog pins

uaanalogDescription
010VbiasR
17r
29g
38b

Chip location

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