J-CIA MOS 6526-8521-(8520) FPGA Replacement

I gotta admit, I never thought this day would come. And yet here we are. The myth becomes reality.

Our story begins on the 30th July 2023 with an announcement:

"J-CIA is finally here!

After a long wait, J-CIA is finally released!

The released version ("J-CIA64") is only compatible with 6526/8521 - CIA.

Amiga compatible version will be released later."

This project was started in 2018 and a like every other retro project of this nature, the post pandemic chip shortage put the breaks on until now. Needless to say I've been waiting patiently for several years for this day to come and the instant I got that notification I was getting my order in, which was just as well as they sold out within a few hours so I am far from the only person who's been awaiting these. Just 6 days later I held in my hands the much anticipated CIA FPGA replacement by Jani Laatikainen.

From a selfish point of view, this brings me another step closer to having a 100% new Commodore 64. Assuming they work (they do) this leaves only the 6510 CPU and the keyboard as the only original parts in my SixtyClone. It is highly likely that we will have Jani to thank for a replacement CPU before long, and Jim Drew's Blingboard may, eventually, solve the keyboard issue.

So what does the J-CIA64 actually do? Unlike the ARMSID and the VIC-II Kawari there's no software configuration for the J-CIA, and it brings no additional functionality, or for that matter, complication and is very nearly nearly as simple as plug and play. It's just a replacement CIA (Complex Interface Adapter). And I'm glad it's finally here.

Let's take a closer look:

Click for an extra large image (may take a few moments to load)

Printing on the ICs isn't crystal clear so thats an iCE40 Ultra™ Lattice FPGA (ICE5LP1 SG48I) sitting on top and four NVT2010 bidirectional voltage level translators to complete the IC collection.

Now here's something of interest: the included instructions advise us we can change mode between 8521 "new" and 6526 "old" CIA by soldering this jumper:

We can solder this jumper, but should we?

For the uninitiated, back in the day, Commodore put CIA chips in the C64, C128 and SX-64 computers and the 1570 and 1571 disk drives. At some point, the original 6526 variant was replaced by the 8521. C64 Wiki tells us:

"later C64-versions may also use the 6526A or the 8521."

As pictured, the J-CIAs are delivered configured for the new CIA variant, the 8521, with a simple solder jumper to configure for the old, the 6526. So the question we must ask, is do we need to solder that jumper?

To answer that I guess we need to know the difference between the old and new variants.

According to Eslapion in this forum post:

"The 8521 is just a HMOS version of the 6526 which dissipates less heat. They are mostly identical from a software and electronics point of view."

Now, I suspect the word "mostly" in that sentence is doing some heavy lifting, otherwise why would Jani give us the ability to choose? That said, Jani's own site states:

"Usually, the mode doesn't need to be changed, most software and hardware are designed to work with both versions."

Eventually, however, I did stumple upon a more comprehensive answer, purportedly from Jani:

"It selects selects/enables different pieces of logic at the core. There is small differences in logic between 6526 and 8521. Timing and everything is the same. I mean bus timing is the same. But, in some cases there is for example 1 cpu cycle difference in timing between 6526 and 8521.”

Make of that what you will. So, for the time being, and because I can't trace an official (from Commodore) list of differences (or any list of differences at all), take the path of least resistance and simply install as is. 

What a magnificent pair!

I then ran a battery of tests, including the Diagnostic Cartridge and a bunch of demos (Lunatico, Star Wars, Kawari Inside, Christmas Mega Demo, Eclectic, Edge of Disgrace, We are Demo, Wonderland XIII) and a handful of games (Sonic the Hedgehog, Commando, Empire Strikes Back, Ghostbusters). Everything ran as expected from my Ultimate II+L with zero glitches. I also loaded a game from my Datasette with no issue. My WIC64 (first time I've used that in a while!)  similarly ran without any obvious problems. As far as I can tell then, these replacements really do just work with no obvious or immediate need to solder the jumper for 6526 operation, though it's still very early days so plenty of time to discover something horrible. I'd also like to add that, unlike the Kawari, SaRuMan and TOLB, I'm bloody delighted to see these come in a sensible colour for my preferred palette.

At €39.95 each these don't come particularly cheap, and when you've factored in shipping from the Netherlands to Scotland, security fees, Paypal fees and "transaction costs" (whatever that is) my wallet took a €90.71 hit for the pair (81.16 GBP or 100.02 USD) from retro8bitshop. However, consider the time and investment Jani has made to make these available. A not inconsiderable amount of money and brainpower is required to develop something like this. Accordingly, I have no problem at all shelling out this sort of sum, not least to help support the developer.

If you decide these are for you, please see the Installation Guide and Disclaimers at Jani's website.

Total Cost Update

If you've read any of my other articles you'll know I've been keeping track of my expenditure, from first getting the SixtyClone board and initial materials, all the way to adding modern component replacements as they become available. As we strive boldly towards my end goal of building an all new Commodore 64, I must add the cost of these J-CIA to my already eye-watering total build expenditure which now stands at:

£1,190.08, or $1,560.89 or €1,399.44 approx.

If you're interested in finding out how on earth building a new Commodore 64 today could possibly cost such a ridiculous sum, please read my other articles.