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SMS and SMS mode output levels

Posted: Thu Oct 22, 2015 11:55 am
by TmEE co.(TM)
Image

And today I finally got the SMS levels checked out. Good chuck went into debugging my flashing hardware which turned out to be faulty in the end.

I got measuremests from SMS1, SMS2, MD1 and MD2.

MD1 and 2 produced same results, SMS1 and 2 I had a bit of different measurement points. The usual point at voltage divider gave huge DC offset on SMS2 and I couldn't do measurements properly because of it so I measured the VDP output instead.

Here's the measurements for MD1 and 2 :

mV, ratio, RGB
0, 0.000, 0
368, 0.387, 99
604, 0.634, 162
952, 1.000, 255

SMS1 :

mV, ratio, RGB
0, 0.000, 0
324, 0.354, 90
620, 0.677, 173
916, 1.000, 255

SMS2 :

mV, ratio, RGB
0, 0.000, 0
364, 0.350, 89
720, 0.684, 174
1052, 1.000, 255

MD1+2 : 0, 99, 162, 255
SMS1 : 0, 90, 173, 255
SMS2 : 0, 89, 174, 255
Linear : 0, 85, 170, 255

Note that the MD measuremests DO NOT match any of the MD native mode output, there is some analog trickery going on or there's separate DACs used for SMS mode output.

Here's the test ROM (it is big because I hacked one game I have been sloooowly working on) : http://www.tmeeco.eu/SMS/COLOR.SMS

Re: SMS and SMS mode output levels

Posted: Thu Oct 22, 2015 3:30 pm
by Sik
TmEE co.(TM) wrote:Note that the MD measuremests DO NOT match any of the MD native mode output, there is some analog trickery going on or there's separate DACs used for SMS mode output.
Suddenly that makes bit 2 of register $00 (the one that trims bits 2-1 from each RGB component) even weirder. Given it's set in mode 4 (if I recall correctly from the Master System docs) I assumed it was just used to let SMS color go through (by overriding those two bits) but...

...actually, that'd make sense if the VDP needs to use different levels in mode 4.