Denial

I realised about a week ago that I've reached a point where I've identified what pretty much all parts of the 6561 die shot do. So I thought I'd have a go at creating a diagram that highlights all of the functional blocks. Here is my first attempt: https://sites.google.com/site/mos6561vic/files/6561...

Looking at the various 6522 implementations (HDL and software) they are all different. How sad. Yeah, I've noticed that as well. I've had a look at various implementations to get inspiration. P.S. peeking into JVic's source code but I could not find sound emulation :-) Yeah, didn't get that far. It...

P.S. can you direct me to a good resource for understanding the VIA, at least the timers... all I have found so far is too technical. I would like to fix the FPGA implementation because it has some glitches. The original preliminary MOS 6522 datasheet on 6502.org has at least one mistake I know of ...

Over the past few evenings, I've been reversing the part of the die shot where the sound clock dividers reside. The following image shows where that area is, highlighted with a pink border: sound_clock_dividers_highlighted.jpg And the following is the logic diagram from my simulation of the reversed...

I have the Kindle version of Brian Bagnall's book "Commodore: A Company on the Edge". That image of the prototype appears in that book, near where it talks about the prototype built by Bob Yannes. Amazon says the Kindle version was published in Feb 2012. Not sure if it was in the earlier e...

I was doing a few searches online and it appears that some people were already aware that the VIC 20 noise LFSR was 16-bit: https://chipmusic.org/forums/post/97950/#p97950 and: https://books.google.co.uk/books?id=UlByDwAAQBAJ&pg=PA71&lpg=PA71#v=onepage&q&f=false That book (published ...

I made a feature request to the VICE emulator asking to implement the newly discovered noise generator. See it here . Cool. I read through it and I think the VHDL makes it clear. My only comment about the description is that saying the taps are 3, 12, 14 and 15 might be misleading to some people. I...

I spent some time this evening reversing what is going on inside that black box, or perhaps I should say what might have been going on if it was connected up. One thing I can say is that it is definitely not purely digital in nature. I can see lots of pull down transistors, which are the transistors...

This doesn't appear to be the case, because the polysilicon output line coming out of each bit of the shift register is an output only. In all cases it is feeding the input of a transistor gate, in fact in each case the output line is feeding two transistor gates. There's no way for a level to come ...

Unidentified at this point of time is what is in the black box, but it does something with all 8 bits of the 8-bit shift register and involves several resistors. Maybe its the start of the DAC part of the circuit. It has an output that appears to mix with the outputs of the other three voices, but ...

setting it to 8 instead of 16, MAKES IT SOUND EXACTLY LIKE A VIC 20 ! Yeee! :D :D :D So now we have the noise generator completely reverse engineered! (at least for emulation/reimplementation purposes). That is great to hear!! :D P.S. Looking at the clock dividers (64, 32, 16, 8 ) makes me think th...

The topic of this post is the 8-bit shift register. Looking at the pink boxes I think I can see the 8-bit shift register used to generate the square waves (it should be the horizontal thing just above the middle of the box). Well, if that is the case, it means an important thing: such 8-bit LFSR is ...

Another update to say that the 8th input to the big NOR gate labelled UNKNOWN in the following diagram of the 7-bit counter... : http://sleepingelephant.com/ipw-web/bulletin/bb/download/file.php?id=3083&mode=view ...is also the signal that controls how often the 7-bit counter increments. I think...

Without further ado, here is the full circuit diagram for the 16-bit LFSR, including the feedback and shifting logic: noise_lfsr_shift_register_full_2.png Obviously you will have to click on that, and then zoom in to see the full sized image. Once you've done that, you'll notice the feedback XNOR ci...

Another update from me that I feel I should pass on immediately. It appears I made a mistake in my earlier posts about the LFSR. It isn't using an XOR but is instead using an XNOR. I will retrospectively correct that post soon, and will put an Update note at the bottom of it to say that I've done th...