Now that the Z80 runs, let's have it generate some video!
SMTs and then some more SMTs
Now, the Video Display Processor (VDP) used in the ColecoVision is the same as the one used in the TI99/4A and many other machines. That's why it's still readily available. There are four variations of this VDP:
TMS9918: this one was only used in the TI99/4A, stay away from this one!
TMS9918A: is a 60Hz variant that outputs NTSC video directly. You could use this variant but it doesn't work with the RGB output and doesn't work with expansion module #1 (Atari VCS module) either! If you do decide to use it, you need to build the video buffer amp (see expansion module #1 blog entry), set Jumper SJ12 and use the composite pin on the MiniDIN jack or populate the RCA jacks. But RGB is much nicer, so stay away from this one too and read on...
TMS9928A: This is the 60 Hz variant that you probably want if you live in the US or Canada. It supports the RGB output and delivers crisp video!
TMS9929A: If you live in the rest of the world, you probably have 50Hz/PAL, and then this version of the VDP is for you.
It's worth noting that "modern" TVs with RGB inputs usually support both 50 and 60 Hz and PAL/NTSC are irrelevant with RGB signals. So you could use both on this board, just make sure you use the proper BIOS to go with the VDP you install!
The VDP used dynamic RAM which needed 3 different supply voltages, which sucks, so we're going to use a different approach. Using three latches we can use a modern static RAM, which works just great! Also instead of generating the 10.738098 MHz needed by the VDP from the 3.57 MHz clock, we're simply going to use a crystal of that frequency.
Before we continue, remove the 10K pull-up R65, because we are going to install the VDP next!
Also, important notice: U4 and U6-U8 must be 74HCT ICs! If you use 74LS, you may get garbled video! (Reason: the timings of LS and HCT are different!)
We start by soldering
R34 4k7 resistor
R38,R66, R67 390 Ohms resistor
C37,C38 56pF
C5 120pF
C76,C83 1nF
U4 74HCT04D (SO14) - it's important that this is a HCT family IC!
U6 74HCT574D (SO20W) - it's important that this is a HCT family IC!
Now carefully solder
U9 62256FP (SO28L)
then
U7,U8 74HCT574D (SO20W) - it's important that these are HCT family ICs!
U13 74LS138D (SO16)
Note: U8 is oriented upside down!
Install a socket for the VDP and install
Q1 10,738098 MHz (HC49/S)
Verify clock frequency 0f 10.739 MHz on pin 40 after allowing hardware to warm up running for 20 minutes.
Note: according to the manufacturer, the clock frequency should be between
10.738098 and 10.739172 MHz. If it is not, you can vary C37/C38 between 18p (higher freq) and 82p (lower freq). I found it doesn't make a difference when using RGB.
Video!!!
Using a scope (triggering on negative HSync, 64uS per scan) you should be able to see something like this:
That's the "C O L E C O V I S I O N" startup screen, you can make out the 12 colored letters of different brightness and that the I'd are a bit more narrow than the rest of the letters. You can also tell the white characters that make up the rest of the screen. Here's a screenshot of line 55 near the last line.
Note: supply current @12V goes up to the 200-250mA range with the 80's NMOS chip, don't be alarmed.
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