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ULA ZX8301 - TV Picture Capabilities

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tcat
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Re: ULA ZX8301 - TV Picture Capabilities

Postby tcat » Mon Dec 04, 2017 1:19 pm

3) 16 color mode. This can be done externally with some logic that has to extract a 10MHz clock from the available 15MHz, and use it to sample and process RGB. An extra control bit could also be implemented using external logic.

Hi,
Dealing with some wave shaping at the Microdrive design topic, also reading a little bit about FPGA and Verilog definition language.

I have some confidence to copy the example of clock divide by `3'. Clock divide by `two' is simple pass through a NOT gate (or a diode?), that is detecting positive edge only.

Code: Select all

module clk_div3(clk, reset, clk_out);
 
input clk;
input reset;
output clk_out;
 
reg [1:0] pos_count, neg_count;
 
always @(posedge clk)
if (reset)
pos_count <=0;
else if (pos_count ==2) pos_count <= 0;
else pos_count<= pos_count +1;
 
always @(negedge clk)
if (reset)
neg_count <=0;
else  if (neg_count ==2) neg_count <= 0;
else neg_count<= neg_count +1;
 
assign clk_out = ((pos_count == 2) | (neg_count == 2));
endmodule

The idea of divide by `3' is to count positive also negative edge. This is a simulation of in
put and output wave.

clkdivby3.png
Clock divide by Three
clkdivby3.png (3.72 KiB) Viewed 268 times

To design `16-bit' color bit VGA signal reprocessor is beyond my capabilities. But perhaps with some help I may start by pinning down all input/output signals needed.
Tomas


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Pr0f
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Re: ULA ZX8301 - TV Picture Capabilities

Postby Pr0f » Mon Dec 04, 2017 1:23 pm

It almost seems to make more sense to replicate the operation of the Aurora but in an FPGA that could be offered as a plug in replacement or plug in card ?

You could design it from the get go for VGA / DVI / HDMI output.


Nasta
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Re: ULA ZX8301 - TV Picture Capabilities

Postby Nasta » Mon Dec 04, 2017 3:56 pm

Pr0f wrote:It almost seems to make more sense to replicate the operation of the Aurora but in an FPGA that could be offered as a plug in replacement or plug in card ?
You could design it from the get go for VGA / DVI / HDMI output.


Exactly, and that is how Miracle's original Masterpiece graphics card started out. It was to be a PCB that plugs into the 8301 socket, plus a 'flying lead' to get to the A18 or A19 address line. The big difference (and the way I'd do a replacement) was that it used a single 128k x 8 VRAM chip in a ZIP package (vertical, like the RAM on GC, SGC, QXL), which dropped into the space between the front of the QL case and the bottom row of the RAM chips on the motherboard. It required 2 CPLDs at the time, these days it would fit one.
The original spec was either original QL resolution or x2 in both directions (1024x512 mode 4 or 512x512 mode 8). These days I'd add 512x256 in 256 colors compatible with Aurora. Someone also mentioned a 16 color mode instead of mode 8... well, Aurora has a 16 color mode at up to 1024x480 (actually 1024x512 is possible by the hardware) but no-one used it as far as I know...



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