F18A MK2

F18A MK2 Isometric

Introduction

The F18A MK2 is the next iteration of the 9918A VDP replacement project I started back in 2011.  When I first set out to make the F18A one of my primary goals was to have the board be the same size as the original 9918A VDP, which means fitting in a 600-mil wide, 40-pin DIP package.  Due to the limitations of PCB manufacturing available to hobbyists at the time, and my lack of experience in PCB layout, I was not able to attain that goal.

Old Problems

The first F18A is almost twice as wide as the 9918A VDP, which makes it a problem when installing in some of the various systems that used the original 9918A/9928/9929 VDP.  Some of the systems placed the VDP very close the perimeter of the main board, or had tall components next to the VDP IC.  Another problem was the VGA header, which protrudes from one side of the F18A board and makes it even wider.

F18A vs 9918A

Here you can see an examples of the F18A not fitting in a Memotech 500 MSX1 computer, and the problem fitting the F18A into the ADAM computer, which required an additional special PCB to offset the F18A.

Memotech 500

F18A not fitting in the ADAM

F18A ADAM Adapter

I made the adapter for the ADAM, but I soon realized I would not be able to make adapters for all the possible systems where the F18A could be used.  It was very frustrating for me, and I’m sure for some users of the F18A.  I also decided to offer two styles of PCB pins to help get the F18A “up and over” neighboring components in some systems, particularly the ColecoVision console.  You can see here the “Tall Pins” vs “Short Pins” options next to examples of the original VDP from various systems with heat-sinks attached.

F18A Pin Options

Availability and New Beginnings

I have always tried to keep the F18A available for the community as long as there was a demand.  Since 2012, 550 F18A boards have found homes in retro-computers of all kinds.  Since this is a hobby for me, when I run out of F18A boards I have to wait for there to be enough demand to have another 100 boards made.  That is about the minimum quantity to keep the costs “reasonable”, so there was always a period between runs when I would not have any F18A boards available.

In May 2017 I ran out of my last batch for boards.  After a month or so I had not received any requests for more boards, and I thought maybe the F18A had run its course and the market was saturated.  Apparently everyone who wanted an F18A had one.  It was not until early August that I received a request for a board, and come November it had only crept up to about 50 requests.

It was at this point that I started to seriously consider the MK2.  Since it had taken almost 6 months to get up to 50 requests, I speculated that it would probably take another 6 months to reach 100 requests.  If the demand stayed linear then that would give me time to produce the MK2.

Compared to 2012, the PCB capabilities available to hobbyists now in 2018 have become pretty amazing.  Specifications that I could only dream about in 2011 are typical now.  I attribute a lot of this to the cell phone industry which is constantly pushing the size of electronics down, and the density of PCBs up.  When PCB houses update their capabilities to support big industries, the benefits trickle down to the rest of us.

Also in the 5 years since I chose parts for the F18A, the cost of FPGAs has come down as well.  Today I can get a Spartan-6 FPGA, with more than twice the capability as the Spartan-3E I used for the original F18A, for less than I can get the Spartan-3E!  The Spartan-6 opens up a lot of capabilities to making the features I need in the MK2 possible.

With the necessary part costs now within the realm of possibility, the PCB capabilities available to make the MK2 board the same size as the original VDP, and the increasing demand for another runs of boards, it was time to make the MK2 a reality.  Thus I began work on the F18A MK2 in earnest.

Apologies

At this point I would like to apologize to everyone who has been patiently waiting (and still have to wait a few more weeks) for an F18A board.  I do not like to release vapor-ware, and until I had a working MK2 prototype I was not going to mention the new design.  If I could not get pull off the new design, I would just make another run of the original F18A board.  So far that has not been necessary, I hope no one is upset, and I really believe the MK2 will be worth the wait.

F18A MK2 Features

This is the candy portion of the story: the feature list.  The MK2 was designed to solve some specific problems, as well as add new features to allow it to be used in more systems (future plans), or as a general purpose FPGA development board.  Two of the most obvious features are the Digital Video output (TBD, most likely DisplayPort) and the size of the board.

I’m sure some people will be unhappy about the Digital Video output instead of VGA, but it was probably the single biggest complaint / comment about the original F18A.  I believe the first thing most people do is get a converter for the F18A to go from the VGA output to Digital Video.  Also, the availability and small size of the cables and connectors made a digital format a simple choice to make.  Modding cases will be easier for people, plus the audio problem is solved for those who can manage to hook up a single wire in their system.  For those who still want or need VGA, Digital Video to VGA converters are available to fill that requirement.

The primary features of the MK2 over the original F18A, as related to the 9918A family of VDPs are:

  • The MK2 is 52mm x 19mm, which means it is the same size as a standard 600-mil 40-pin DIP socket.
  • No pin-options are required for various systems.
  • The MK2 outputs native Digital Video via an on-board connector.
  • The MK2 has an audio input pin that can be used to inject the host computer’s audio into the Digital Video signal.
  • The MK2 powers-on much faster and should not cause problems for computers with short Power-On Reset circuits.
  • Dual-voltage level shifting is used for all host-system inputs and output pins.

The MK2 will run the same core as the original F18A, and I am also committed (and still working on) fixing certain firmware problems with the original F18A.

New features of the MK2 that will make it possible to replace other VDPs in the near future:

  • The MK2 has 512KiB of on-board SRAM that will be usable as VRAM.
  • There are 14 general purpose 3.3V IO pins available, in addition to the standard 9918A host interface pins.
  • Mode-1 pin for 9938-compatible retro-fitting in a 9918A-based system.
  • Standard JTAG interface.
  • More than double the FPGA resources in the Spartan-6 LX9 over the Spartan-3E used in the original F18A.

Some trivia and technical-specs for the curious:

  • The MK2 schematic and PCB was created with KiCAD V4.0.7
  • All prototype PCBs were made by OSHPark
  • The MK2 requires 5/5-mil trace/space for escaping the FPGA and routing the SRAM
  • The FPGA is a 255-pin BGA with 0.8mm pitch
  • All the capacitors are 0402 ceramic (that’s 1.0mm x 0.5mm!)
  • There are 6 level-shifters on the board, 5 are on the bottom and are 1mm x 2mm 8-pin, 0.5mm pitch BGA
  • The voltage regulators on the board are the same as some used in the iPhone-6s, only I used the larger SOT-23 package
  • I assemble the prototype boards myself using a home-made reflow oven for the BGA ICs, and hand solder the reset with hot-air
  • I use a 70x stereo microscope for soldering / assembly
  • The MK2 uses about 120mA when operating normally

Here are some renderings and photos of the actual prototype:

F18A MK2 Top

F18A MK2 Bottom

F18A MK2 Proto Iso

F18A MK2 Proto Bottom

F18A MK2 Proto vs 9918A

Status

As of this writing, the MK2 is currently in prototype testing.  Almost all the new features are tested and working, and I am currently waiting for the revision “H” (yes, that is EIGHT prototype revisions) PCBs to arrive from OSHPark.

I would like to take a moment to say “thank you” to OSHPark!  They are an awesome service and have made this whole project possible thanks to their affordable prices, easy to use website, and great PCB capabilities.

Once the final PCB prototype is proven working, I will email everyone on the waiting list and open my web store for pre-orders.  Based on the actual orders made by people, I will place the order with the PCB manufacturer to have the MK2 built in quantity (the scary part).  Last time the manufacturing took about 2 or 3 weeks.  Once I get the boards I will program, test, and ship them out.

I expect to get the lasted PCB back in about 10 days, and I will do the assembly and testing.  So if everything to according to plan, I hope to be able to place the big order sometime in July.

Since I have gone ahead and announced this new board, I will also be trying to keep the status updated more regularly.  As of right now I plan to post the on-going status in a thread on the Atari Age forums, in the TI-99/4A subforum (https://atariage.com/forums/forum/119-ti-994a-development/)

Atari Age is my primary retro-computer hang-out, and there is a great community and a lot of information there.  I highly recommend you check it out if you have any interest in retro-computing of any kind.

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