492 TT06 Grab Bag :: Quicker, easier and cheaper to make your own chip!

tt06-grab-bag GDS layout showing digital block, 4 DACs, and 1 inverter

What is this thing?

A simple analog/mixed-signal project I created when invited to participate in the first round of Matt Venn's Zero to ASIC Analog Course beta, and ultimately submitted to TT06.

It comprises the following:

A bog-standard CMOS inverter. That was my very first custom layout attempt.
A digital block that generates a few basic 24b-colour (RGB888) VGA test patterns.
Analog RGB outputs (running digital block VGA outputs through 3x 8-bit R2R DACs).
An extra 4-bit R2R DAC.

VGA test pattern outputs

The design's main purpose is to generate VGA test patterns that will hopefully look as good as these simulations:

Simulated VGA outputs, XOR pattern and RAMP pattern

The left-hand pretty pattern is "MODE_XORS" (ui_in==8'b0011_0000) while the right-hand gradients pattern is "MODE_RAMP" (ui_in==8'b0001_0000).

Notice there is some horizontal smearing (more exaggerated in the right-hand image of the red/green mixes). The outputs might look better, or could look worse. There could even end up being weird banding or noise. Let's wait and see!

The internal R2R DACs for each of the RGB outputs just go directly (unbuffered) to the analog output pins, where they are subject to the loading of the TT06 analog mux (estimated to be about 500Ω and 5pF). This combination means their slew rate is expected to be pretty bad (at least by VGA timing standards): On the order of 240~360ns (or 6~9 horizontal pixels) going from 0V to full 1.8V.

In a future design I plan to implement internal buffering to help mitigate some of the TT analog mux load.

NOTE: You will almost certainly need some sort of output buffering between this design and a VGA display, because the design outputs a high-impedance (~10kΩ but maybe a little worse) 0~1.8V range, while a VGA display expects 0~0.7V at 75Ω. Read 'How to test' for more info.

CMOS inverter

Pretty simple:

Its input is uio_in[7] (bidir 7).
Its output goes to two places: ua[3] (analog) and uio_out[2] (digital).
I would expect its digital output performance to be better, because it has more buffering (and less loading) along the TT digital mux, than what it does on the TT analog mux.

The graphs below show that the simulated analog output is expected to be stable (enough) within 10ns. This relatively poor performance is characteristic of the TT analog mux loading. I expect bigger transistors could drive this harder and make it faster.

Xschem simulation of my CMOS inverter

Extra 4-bit R2R

I took one of the 8-bit R2R DAC layouts and copied it, pulling the 4 LSB to GND, and connecting the 4 MSB to spare bidir inputs: uio_in[6:4]. This DAC outputs via ua[4].

How it works

TBC!

Select from a few simple test patterns in the VGA controller by having different ui_in values asserted while coming out of reset. the VGA controller digital block generates 8-bit digital outputs per each of red, green, and blue channels. These go into 3 basic RDACs to generate analog voltage outputs on ua[2:0] ({B,G,R}) in the range 0-1.8V (probably ~10kΩ impedance).

How to test

TBC!

Supply a 25MHz clock
Set ui_in to 8'b0001_0000
Assert reset -- NOTE: I didn't put a synchroniser on it, so it might (?) do a dirty reset -- if that happens, it could be worked around by slowly/manually clocking around the reset pulse, I guess.
With a scope, trigger on the uo_out[3] rising edge (VSYNC) and hopefully see ua[0] ramp from 0V to 1.8V within 10.24us
With this mode (as selected in step 2 above), ua[1] will also ramp, but per line (instead of per pixel), as will ua[2] (per frame).

Other notes for testing:

External buffering (opamps?) for analog RGB outputs, to match 0~1.8V@10kΩ to 0~0.7V@75Ω
Digital block's mode selection is asserted via ui_in during reset
For safety, initial test should be done with no analog output loading, and with all of ui_in pulled low (which selects pass-thru mode AND ensures all DAC inputs internally are low, so hopefully no current).
RGB222 digital outputs compatible with the Tiny VGA PMOD.

External hardware

This is if you want to see an actual analog VGA display:

10MHz-capable (or better; preferably 25MHz) opamps on each of the R, G, B outputs, to both make them into low-impedance (matching 75Ω typical VGA termination), and also to level-shift from 0~1.8V to 0~0.7V.
Optionally the Tiny VGA PMOD plugged into the dedicated output port (uo_out).

Come back later and I'll have a better explanation of how to hook up to a VGA display.

#	Input	Output	Bidirectional
0	mode[0] / dac_in[0]	r7	vblank_out
1	mode[1] / dac_in[1]	g7	hblank_out
2	mode[2] / dac_in[2]	b7	inv_dout
3	mode[3] / dac_in[3]	vsync	dac4_in[4]
4	mode[4] / dac_in[4]	r6	dac4_in[5]
5	mode[5] / dac_in[5]	g6	dac4_in[6]
6	mode[6] / dac_in[6]	b6	dac4_in[7]
7	mode[7] / dac_in[7]	hsync	inv_in

`ua`	`analog`	Description
0	5	r_out
1	0	g_out
2	4	b_out
3	1	inv_aout
4	3	dac4_aout
5	2

492 TT06 Grab Bag

492 : TT06 Grab Bag