If they work okay, I should have a few spares available in the new year. I've ordered enough parts and boards to make twenty.
In case you can't see the image for some reason, here's a link to it: link to .gif image of the PCB, hosted on my website
In case you can't see the image for some reason, here's a link to it: link to .gif image of the PCB, hosted on my website
You know you can get a 4 Mbit SRAM in SOIC-8
Yeah, I've used SPI and I2C RAM, but any serial RAM is way too slow to act as a graphics buffer for VGA or composite video on an AVR. Also, that chip is 3.6V maximum, so no good for the Mega, unless you also add level converters.
What is going to be the memory chip for the board?
It's the one described in Andy Brown's article about the design - an Alliance AS7C4096-12TCN. It's a 12ns 512k static RAM. Here's a link to the datasheet (.pdf): https://www.mouser.co.uk/datasheet/2/12/as7c4096a_v1.2-1288189.pdf
The Arduino Mega, Mega2560, and the Mega ADK all have the option to add extra RAM that operates just as fast as the chip's internal RAM. The chip can only address 64k of RAM at once, so the half-meg is split into 8 banks. Some of the chip's memory is unusable because it overlaps the Mega's own internal RAM space, so you actually get 56832 bytes per bank - or 454656 bytes extra in total.
They're built, tested, working fine.
There are LEDs to indicate when it's reading and writing from/to the expansion RAM. I chose resistors for the LEDs so they're not too bright - I hate being dazzled by super-bright LEDs on circuit boards.
As you can see, it plugs into the double-row connector on the Mega. Pins 22 to 43 are used for the address bus and data bus of the RAM, and to drive the bank selection. Pins 44 to 53 aren't used by the expansion board, so they're reproduced in the on-board connector, along with the GND and 5V pins.
I made it the same colour as the Mega, and I think it fits quite neatly, without covering up the reset button, the ICSP connector, or anything else you need to access.
Nice job.
Thanks.
I noticed an error on the silk screen - I've got the label for pin 48 twice, once for the real pin 48, and again for what should be pin 49 :o
I've used a craft knife to scrape away the bottom left portion of the offending 8s, to make slightly wonky, but passable, 9s 
If anybody wants one, please email or PM me. I've made twenty and all are working. I'll keep maybe three for myself, so I have about seventeen to sell. The parts and shipping weren't cheap, so the boards have cost me about twenty pounds each. Labour for the PCB design, and for building and testing the boards, was free of course!
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