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https://github.com/hkzlab/c64_250407_sram_adapter
SRAM Adapter for the C64 250407 board
https://github.com/hkzlab/c64_250407_sram_adapter
Last synced: 6 days ago
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SRAM Adapter for the C64 250407 board
- Host: GitHub
- URL: https://github.com/hkzlab/c64_250407_sram_adapter
- Owner: hkzlab
- Created: 2025-01-26T10:02:37.000Z (16 days ago)
- Default Branch: main
- Last Pushed: 2025-01-26T11:22:53.000Z (16 days ago)
- Last Synced: 2025-01-26T11:25:51.960Z (16 days ago)
- Size: 1.18 MB
- Stars: 0
- Watchers: 1
- Forks: 0
- Open Issues: 0
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Metadata Files:
- Readme: README.md
Awesome Lists containing this project
README
# C64 ASSY 250407 SRAM Adapter
## DISCLAIMER
Any use of this project is **under your own responsibility**.
You will be responsible of checking the correct construction and functionality of your board.
By using this project You will agree that I cannot be held responsible if it will destroy any of your devices, damage your computer, burn down your house or whatever.
**WARNING**: This is compatible with the 250407 board only!!!
## Introduction
This board implements a static RAM replacement for the DRAMs on a C64 with a 250407 motherboard.
I wanted to both reduce power consumption, and get a fix for the [VSP bug](https://www.pouet.net/prod.php?which=61024#c637759).
You will have to socket your DRAM ICs to install this board.
### Ordering the boards
You can order the board for this project at [PCBWay](https://www.pcbway.com/project/shareproject/C64_250407_SRAM_Adapter_9502264b.html) and support me or download
the gerbers and have them printed wherever you like!!!
## Features
- You can choose to use SMD or DIP components for the logic and memory ICs
- Once the DRAMs are socketed, the board is easy to plug in and remove
- If your C64 lacks C203, a capacitor can be fitted on this board without having to further modify your motherboard
## Hardware
The board was designed with [KiCad](https://kicad.org/) EDA.
### Bill of Materials
| Component | Qty | Type / Value | Notes |
| ---------------------- | --- | -------------------------------- | -------------------------------------------------------------------- |
| C1, C2, C3 | 3 | 100nF ceramic cap, SMD 1206 | Decoupling caps, rated at least 10V. |
| C5 | 1 | ~82-820pF ceramic cap, THT 2.5mm | Optional, install either this or C4 if C203 on mobo is missing |
| C4 | 1 | ~82-820pF ceramic cap, SMD 1206 | Optional, install either this or C5 if C203 on mobo is missing |
| U1 | 1 | 74HCT32, DIP14 or SO14 | Can be replaced with an ACT/ALS/LS/F. Do NOT use an HC. |
| U2 | 1 | 74HCT573, DIP 20 or TSSOP20 | Can be replaced with an ACT/ALS/LS/F. Do NOT use an HC. |
| U3 | 1 | 128Kx8 SRAM, DIP32 or SOP32 | I tested ICs between 75ns to 100ns without issue. 628128 pinout. |
| Headers | 1 | 40p 2.54mm machined header strip | Cut it into 4x8p, 1x2p and 5x1p pieces, then solder on the underside |
C4/C5 adjust `/CAS` timing to make the board play nice in C64s that do not have C203 fitted.
Value of C5/C4 is non critical, I had good luck with 300pF and 100pF caps. Note that I could also make do without the cap entirely
by replacing U1 (the '32 gate) with a schottky based one, so LS/ALS/F family.
### Fixes
#### Rev. 1
Rev. 1 lacks the space to solder C4/C5, you can just bodge a cap on the first two pins (`GND` and `/CAS`) of the bottom right header.
### Known Issues
I could not detect stability issues, the only problem I found is a graphical glitch on the title screen of ZetaWing 2,
where a single lines of pixels is flickering.
Note that this same issue **happens with other SRAM mods** (meant for shortboards) bodged to work on this machine, and disappears when using real DRAM ICs.
This seems a quirk of my VIC-II, a 6569R3. I modified the motherboard to support an 8565 and the issue disappeared (tested with an 8565R2).