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artemisamsx / artemisa

Schematics, PCB designs and HDL simulations to build a 8-bits MSX computer
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External VDP must not be connected to GND #124

Closed apoloval closed 4 years ago

apoloval commented 4 years ago

After many tests, several hours in front of the oscilloscope, we have found out that we cannot connect external VDP input to ground. This is causing a very subtle problem when VDP is configured in screen 2 with borders in transparent color. The following Basic program can be used to reproduce the problem:

10 SCREEN 2
20 VDP(7)=0
30 A$=INKEY$
40 IF A$="" THEN 30

The line 20 just sets the frame border color to 0 (transparent). This causes a distortion in the CVBS signar that some TV sets cannot recover from. Some games use to left the frame border color as transparent, since it should be a fallback to black according to the datasheet. Arkanoid is one of them. Running these games caused the synchronization signal to be lost.

The solution is to leave EXTVDP signal unconnected.

apoloval commented 4 years ago

Fixed in c5b833b742c23c2ca5edafb00c0926f8561686bb.