Comments about components and optional parts:

Q: Do I need VGATE to run this computer?
A: No. The only chip to buy/program is  TK-II (PIC16F184) – keyboard interface and reset delay. To enable video connect pin 1 and 3 of VGATE using any resistor you have.
enable video without vgate

Q: Can I boot/test computer without TK-II?
A: Yes and No. Computer may start without it. To start the computer, ground /RST for a moment. Computer will start, but there is no keyboard.

Q: Atari chips are hot!
A: It is normal- ancient technology. Try to reduce +5V rail voltage a bit (mine is working fine at 4.6V. Some adjustment may be needed to compensate for voltage drop  in video circuit).

Q: You have PALCE programmer, I don’t have.
A: Convert PAL fuse file to GAL fuse and program GAL chip.

Q: I have only the original PAL (atari chip) for MMU. What? How?
A: Original MMU chip is enough to run Atari. No ROM basic in this version. If you want to use BASIC, use BASIC /CS to connect additional ROM or ROM PAGE. Use your brain. Or just use cartridge with basic.

Q: Where can I get a SRAM chip?
A: It is an old PC cache SRAM chip. Look in e-waste for old 486 or maybe Pentium-1 computers. Bigger chips are 64K, smaller-32K. Speed does not matter. Or buy it from Aliexpress pull-outs.

Q: 128K?
A: First, buy (or find) 128K SRAM. Next… I still don’t know. 🙂 PCB can accept 128K SRAM and maybe 256K (I never seen this size).

Q: I see you use 74VHC08, 74AHC74 and 74VHC00 chips. Why?
A: My chip seller is very slow and I had PCB on my hands. So, I salvaged these chips from some old equipment.

Q: Where to get those SMD resistors and caps? My component seller is selling only 1000x.
A: Buy 0805 resistor and capacitor set in China.

Q: L3 and C60 is empty in your PCB?
A: There are spare places for different sizes of capacitors and inductors. PAL inductor may be not available at your dealer, so use the nearest available and change capacitor value. Or use a bigger inductor package. Or use another inductor and variable capacitor. Or use a “thru hole” inductor instead of a variable capacitor. Adjust this place for the best “jail bars” effect reduction.

Comments about usage:

(Me): For testing, I modified MMU (PAL, PALCE or GAL) equations to use the same ROM chip for OS and BASIC. Basic now works, but I have a question- in the original atari, “option” key disables basic. In the original 1088XEL there is no basic at all- MMU pin is floating and everything is done with U1MB. In my version, Option key is not disabling basic!. Also, I can not start tape loading with the “START” key. But, if I connect GTIA’s START or OPTION pins to ground, the computer starts tape play and/or diag properly. The problem is with TK-II PIC software.
(mytek): What you are experiencing is a lag caused by the initialization of the PS/2 keyboard when you first activate power. During that power-up cycle, the keyboard is unable to respond to key presses, hence the reason why the START and OPTION keys are not seen in time by the Atari OS to take affect. The U1MB 1088XEL/XLD version BIOS takes this into consideration, and holds off the start-up sequence until the keyboard has had time to initialize.

(Me): These keys are working fine in games or OS diagnostics. I can see that OPTION and START lines go low to late on boot.
(mytek): Yes there is nothing wrong with TK-II’s interpretation of these keys.

(Me): Maybe there is a special key press to enable this boot option from the PS2 keyboard?
(mytek): There is a way provided to always force BASIC off on start-up. CTRL+ALT+O will toggle this feature ON or OFF. ALT+O will show it’s present state. This is a non-volatile setting. Also on the V2.4 TK-II firmware there is a start-up delay option (CTRL+ALT+D to toggle it, and ALT+D to show its present setting) that holds off the system from booting by holding reset low for an extended period while the PS/2 keyboard fully initializes, thus having any held key be recognized by the OS. This too is a non-volatile setting restored upon power-up.
The only down side to this feature, is that the video output also doesn’t get initialized and will do some strange things visually during that sustained reset. This won’t hurt anything, but it will display some crazy colors at times.

Leave a Reply

Your email address will not be published. Required fields are marked *