How to use I/O pins of an SRAM dip package?

Other Hardware Storage
1

I've been looking for SRAM dip packages on eBay for a while, and I've found many, but they all use I/O rather than specific pins for inputs and outputs. I'm unsure how to switch a pin from an input to an output on the fly or how I should be timing the switch.

I was hoping to find something more like this (1) FUJITSU MB81C67-35 MB81C67-35P 16Kx1 SRAM nMOS 5V 20 PIN DIP | eBay but with 8 bits rather than 1. It has a pin meant for input and another for output, which just makes things easier.

I'm leaning towards finding a chip that has full 8-bit words, but also has dedicated in-out pins (if possible). I'm okay with buying a chip that uses I/O pins, I'd just like to know how to use them before spending money on it.

I haven't had much luck with Google. Can anybody help?

2

Isn't this the purpose of data sheets?

3

RAM modules connect to a bus and thus can not have separate I/O pins, except with SPI MISO and MOSI.

You can have any number of RAM chips on a bus. All components, including the controller, have configured their data pins as inputs. On a read request the controller activates one chip for reading what enables its outputs and reads the data from the bus. For writing the controller makes its pins outputs and selects the target chip for a write cycle.

Arduino Mega comes with a RAM interface that handles all the I/O pin switching. See 9. External Memory Interface in the ATmega 1281 data sheet.

4

Can you explain what you mean by this?

5

You could use an I2C SRAM chip and let the Wire library worry about how to do the I/O. 2kx8 and 8kx8 are common sizes. you hook them up to the SDA and SCL pins and access them by address.

6

Probably not. Please post a link to the actual data sheet.

Memory chips like that have a bunch of address pins, which you use to select an address to read or to write, one or more bidirectional data bus pins, and some control inputs that are manipulated to accomplish the actual reading or writing.

Here is a link to the data sheet for a closely related Fujitsu chip M81C68A, which is 4K x 4 bits: https://download.datasheets.com/pdfs/2016/2/2/4/56/54/168/fmi_/manual/fjsus04926-1.pdf

This is very old technology, so anything you buy on eBay is probably recycled from discarded PCBs, and probably isn't guaranteed to work.

7

One of the many things you have not told us is what you plan to do with this memory.

If it is to give you more room for variables and arrays the only processor you can use for this is a Mega. All the others have no way to extend the memory map.

If you want it for storing specific things in like a sound sample buffer, then an SPI interface can be used and you will have to use a small piece of code to read and write to this memory.

8

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