张小哥

【FC add-on】External RAM module

Overview

Some cartridges (such as NROM, UxROM, CNROM, AxROM) do not include or support PRG RAM, that is, the $6000-$7FFF area. This module will add PRG RAM support to this area.

74xx139: It is recommended to use 74LS139, which is the decoder marked "U3" on the host, because when ROMSEL and M2 change level, M2 changes level first, and then waits for about 33ns before ROMSEL changes level. The feature of using 74LS139 is to match the 1Y3->2G delay with ROMSEL.
R1: Allows the CE of PRG RAM to be set to a high level in the power-off state to protect the archive data stored in PRG RAM.

If you need to support archive, you need to solder VBAT, D1 and R1, and D2 solders the diode. Otherwise, you do not need to solder VBAT, D1 and R1, and D2 is shorted.

Please also note that this module can only be used for development.

Design Files

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It is your responsibility to test the circuit yourself and determine its suitability for you. EEWorld will not be liable for direct, indirect, special, incidental, consequential or punitive damages arising from any cause or anything connected to any reference design used.

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Update:2025-05-06 20:10:26

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