🇦🇺 Commodore VIC-20 Internal RAM Replacement: Part 2 (32KB Progress Update) [#TCE0696]

Thanks to #PCBWay for their ongoing support! https://www.pcbway.com Ad-free version on my Patreon ...   / thecluelessengineer   It's been a year since we last had a look at my attempt at an internal RAM upgrade for the Commodore VIC-20 - my 8KB design worked fine but I ran into issues trying to extend it to 32KB and create a FAT VIC. I'm almost there ... one final issue which I should have resolved in the next version. The goal of this project: [1] Understand how the VIC works a bit better - best way is to get your own hands dirty and make the mistakes yourself; [2] Contribute something possibly useful to the VIC community that might be of interest to someone; [3] Have a way to replace possibly-faulty internal RAM; [4] And, make a FAT VIC without needing an external cartridge and that you can switch off without opening the case. A big thank you to ‪@DaveCurran‬ of Tynemouth Software for his advice pointing out where my design had an error and some simplification of the logic - check out his blog here: http://blog.tynemouthsoftware.co.uk The current design is, ahem, "working" but I made an error in how the A13 & A14 address lines were being handled. Dave advised that the screen corruption was likely being caused by the A13 & A14 address lines being of random value during the VIC's (the 6560/6561) access to the RAM. These address lines should be low as the VIC only accesses BLK0. The CPU and the VIC share the RAM and access it on alternate states of the SΦ2 clock ... when it's high, the CPU is accessing and A13 & A14 are valid. When it's low then they should be low - if either or both are high then the VIC is accessing random garbage (i.e. outside of BLK0) which is what we see. I was able to confirm this by pulling A13 & A14 on the RAM low which cleared up the problem for the BLK0-only expansion. I need to add logic to properly handle the A13/A14 addresses depending on the state of SΦ2. I have corrected this in the design and made other improvements which I should be able to test after the updated PCBs are fabricated. I've switched to using a PLD (GAL) otherwise there'd be a few discrete logic ICs required ... if you can program an EPROM then it's very likely you can program a GAL, and having a GAL makes it far simpler to adjust the logic if it's not working! My Commodore VIC-20 Github: https://github.com/BrettHallen/Commod... 00:00 ... Into 04:30 ... The pre-upgrade VIC motherboard 08:10 ... My A13/A14 handling error 10:50 ... How the current design works 12:45 ... Grounding A13/A14 for BLK0 expansion 13:30 ... Channel thanks & outro Part 1 video:    • 🇦🇺 Commodore VIC-20 Internal RAM Replaceme...   -------------------------------- Check out my Patreon for regular blog-type updates between videos - there is a free membership tier and two paid tiers (US$2 and US$5). There is also a one-week trial for the paid tiers so if you don't think it's worth it, then cancel ... no hard feelings ... I appreciate any support, even if it's just subscribing to my YouTube channel!   / thecluelessengineer   -------------------------------- Outro music: "You Can't Kill The Boogeyman" (White Bat 49) by Karl Casey @ White Bat Audio https://karlcasey.bandcamp.com    / @whitebataudio