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Gigatron emulation
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H.G.Muller



Joined: 10 Mar 2006
Posts: 21997
Location: Amsterdam

PostPost subject: Re: Gigatron emulation    Posted: Sat Dec 09, 2017 3:29 pm Reply to topic Reply with quote

The palette is such that each pair of consecutive bits specifies a primary color, at 0, 33%, 67% or 100% intensity. The two lowest bits are for red. (And I suppose the next two bits green, but I ever actually verivied that with Marcel.) There are simply resistors R and 2R conecting the output bits with the VGA pin for that color, to make a 2-bit DAC.

The sound comes from the XOUT port, which is clocked by a SYNC bit of the OUT port. For the rest I think it is purely a software matter; the interpreter would send new output to XOUT every HSYNC pulse (so at 31kHz). I have no idea how it decides what to send.

The input is complex. It seems to be a shift register, clocked by the HSYNC output of the video OUT port. It seems that game controllers are such serial devices, and need to receive a parallel-load signal to get their button states in their shift register. This parallel-load signal is sent to them from the VSYNC pulse. So 8 scan lines after the (end of?) VSYNC the full button info would be readable from IN. No idea how the bits are assigned to buttons.
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Display posts from previous:   
Subject Author Date/Time
Gigatron emulation Martin Sedlak Sat Dec 09, 2017 2:54 pm
      Re: Gigatron emulation H.G.Muller Sat Dec 09, 2017 3:29 pm
            Re: Gigatron emulation Martin Sedlak Sat Dec 09, 2017 4:08 pm
                  Re: Gigatron emulation H.G.Muller Sat Dec 09, 2017 4:37 pm
      Re: Gigatron emulation Martin Sedlak Sat Dec 09, 2017 7:22 pm
            Re: Gigatron emulation Martin Sedlak Sun Dec 10, 2017 1:57 pm
            Re: Gigatron emulation Martin Sedlak Sun Feb 11, 2018 5:46 pm
      Re: Gigatron emulation Rémi Coulom Sat Dec 09, 2017 10:30 pm
            Re: Gigatron emulation Martin Sedlak Sat Dec 09, 2017 10:50 pm
                  Re: Gigatron emulation H.G.Muller Sat Dec 09, 2017 11:03 pm
                        Re: Gigatron emulation Martin Sedlak Sat Dec 09, 2017 11:10 pm
                              Re: Gigatron emulation H.G.Muller Sat Dec 09, 2017 11:35 pm
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