DECA Retro Cape

IMPORTANT: this PCB is unusuable due to an incorrect spacing between the connectors!!!

This project is a plug-in PCB for the Arrow DECA FPGA board to extend it with the following features:

• 2 16Mx16 SDRAMs

  Compatible with the Mister SDRAM xsds.

• 1 PS/2 Keyboard interface

  Mouse will not work, because it's a unidirectional interface.

• 2 Atari Joystick DB9 ports

• 1 general purupose PMOD connector

Like the SDRAM xsds board, I'm using a 5V to 3.3V LDO instead of relying on the 3V3 supply of the DECA board itself.

Status: sent to JLCPCB. Not tested...

Various Notes

The information below is not always relevant, but just stuff that I gathered while working on this PCB...

Different Mister SDRAM Versions

Mister Addons are all designed with Altium. If you don't have a license, you can use the Altium on-line viewer to check out schematics and PCB. I've made a screenshot of some of them.

There are 2 main variants of SDRAM models:

1. With dedicated DQML/DQMH/CKE pins

2. With DQML/DQMH connected to A11/A12 and CKE strapped to 1.

   A11 and A12 are only used during RAS cycles, not during CAS cycles. Conversely, DQML and DQMH are only during CAS cycles. So you can multi-purpose 2 pins, and thus save 2 pins on the connector.

   By strapping CKE to high, you can't use certain low power modes of the SDRAM, but that's not a major issue for a Mister-like design. However, the SDRAM datasheet also says that CKE must be low which VDD is being powered up, so that might be a violation depending on the overall power sequencing.

Because the second variant requires 3 pins less on a connector, it's now the most used option.

• SDRAM SRAM v1.3

  • 1x 16x16Mb SDRAM and 1x SRAM 8x2Mb SRAM.
  • Uses CKE to select between SDRAM and SRAM: CKE is connected SRAM.nCE.
  • Variant 1 connector.
  • Not used very often.

  

• SDRAM uni 3.2U

  • 1x 16x16Mb SDRAM
  • Variant 1 connector.

  

• SDRAM xs 2.2

  • Extra Slim (more compact than uni)
  • 1x 16x16Mb SDRAM
  • Variant 2 connector.

  

• SDRAM xsd 2.5

  • Extra Slim, Dual-chip

  • 2x 16x16Mb SDRAM

  • All pins are shared between the 2 devices (so no double BW).

  • Variant 2 connector.

  • nCS for SDRAM2 is inverted version nCS of SDRAM1.

    This works because SDRAMs has 2 NOP commands: when nCS is high, all commands to the SDRAM are NOP, and when nCS is low, there's still 1 combination that's also a NOP.

  

• SDRAM xsds 2.9

  • Extra Slim, Dual-chip

  • Logically identical to SDRAM xsd.

  • Uses DE10-Nano GPIO1 pin 11 (5V) in combination with an LM1117-3V3 LDO to power the SDRAMs for higher noise resistance, instead of using GPIO1 pin 29 (3V3), which was used in all previous designs.

    

  

Ferrite Bead Info

• Analog Devices - Ferrite Beads Demystified

• Choosing and Using Ferrite Beads

• TI - High-Speed DSP Systems Design- Reference Guide

• Altium - Everything You Need to Know about Ferrite Beads

  Argues against its use.

References

• Hardware addons for MiSTer - SDR SDRAM daughter board

  Discusses different board versions and SDRAM types, clock speeds.

  This repo also contains the Altium designs for the different add-ons. They can be viewed with the Altium Online Schematic and PCB Viewer.

• Addon Board Repo on GitHub

• SDRAM Board Wiki

• AS4C32M16SB-6TIN datasheet

  • AS4C32M16SB-6TIN on Digikey
  • AS4C32M16SB-6TIN on Mouser

• Altium Online Schematic and PCB Viewer

• Mister dual-SDRAM Reliability discussion

• SDRAM Termination discussion

• SDRAM Termination discussion

• SDRAM Termination question on Stackexchange

• Discussion about SDRAM xsds v2.9

• Waveshare VGA PS/2 Board