I’ve been working on a few new boards, mostly for my own tinkering, one of which is this Serial-interface EEPROM Board. It’s a dual-interface – SPI and I2C – board that can be populated on the TOP side for SPI hardware or populate the bottom side for I2C hardware.
I plan to build one of each flavor of this board. For the SPI side, there are many SPI EEPROM chips available in SOIC8 packages. For the SPI side of this board, I intend to use the M95M02 from ST Micro, which holds 2-Mbit each. There is room for (8) of these EEPROM chips on this board, as well as a 3:8 SPI-interface decoder. The decoder allows for true 4-wire SPI interface to the entire array of EEPROM memory. This also makes things nice from a firmware and microcontroller IO perspective.
For the I2C side, I wanted to have the same number of chips and same overall storage capacity so that the only difference would be which interface – SPI or I2C – was chosen. I ran into a road block with this however, as the I2C variant of the M95M02, the M24M02, only had one address pin, which limits the number of unique addresses to only 2. Also, all other I2C serial EEPROM’s I could find in a SOIC8 footprint, albeit smaller capacity, only had two address pins. And this limits the number of unique addresses to 4 without building a more complex hardware addressing scheme. This would have been a bit more complex than the decoder scheme I used on the SPI side of the board, so I decided a 4-up array or I2C EEPROM would be sufficient.
This was a fun little mini project that took the better part of a day starting from scratch and ending with a PCB order from OSH park for a 2.25″ x 1.00″ x 4-layer board for about $23 shipped. With OSH park, you always get 3 copies of your board with each order, so the per-board cost is a little under $8! Wow, I wish this service had been available when I was in college… Stay tuned for more designs to come.
The KiCAD design files and gerbers can be obtained from my bitbucket repository here. Note: Rev1 schematic corresponds to the boards shared on OSH Park. I made a slight change on Rev2 to eliminate some unnecessary resistors.