This was my very first piece of Electron hardware. It looks a bit odd, with the soap-on-a-rope arrangement of the MicroSD reader. There's a story behind that - the board was never actually meant to be a card interface at all. Orginally the project was simply to interface an EPROM chip with the Electron, nothing more. After laying out the PCB I had a few spare I/O pins on the CPLD, so I ran those out to a header for debugging purposes.
Then I came across a post on the StarDot forum from the hugely talented David 'hoglet' Banks, maintainer of MMFS. MMFS is a piece of software that permits the use of memory cards on Acorn 8-bit machines. Up to this point MMFS generally required a 6522 VIA chip or something similar to work; not an issue on the BBC Micro, which has a 6522 driving its user port. But a major complication on the Electron, where a card interface would have to provide the necessary hardware.
But David had written some experimental code to allow MMFS to drive a cheap 'arduino' type card reader board from the printer port on an Acorn Plus 1 expansion. This didn't need a 6522, just two output and one input lines which the Plus 1 mapped to bits in a couple of registers. Nice and simple.
I realised right away that I could emulate the Plus 1 printer port on my simple EPROM board using it's Xilinx CPLD chip. The necessary verilog code mostly existed already as I'd done something similar on an earlier aborted board for the Commodore Plus/4.
And lo, it worked. The EPROM board became a card interface, and I posted about it on StarDot. Then people started asking if they could buy one. So I tidied up the design a bit, and the Mk1 MicroSD interface became a thing.
elksd.zip - a collection of Electron games, just unzip and copy to a MicroSD card
See the Mk3 Interface page for details.