Retro CPC Dongle – Part 18

Since my last post, I’ve been beavering away on the SPI interface between the Atmel supervisor chip and the FPGA. The SPI interface is almost ready to share, but not quite. In the meantime, I’ll share the little side project I’ve been working on, replacing the NAND raw flash with an eMMC chip on the CPC2.0 board.

I wrote about raw flash and the challenges of writing a flash translation later in part 16 of this series. After some research, I concluded that the eMMC interface looked exactly like the much more common SDCard interface, albeit that the interface can be run with an 8-bit width. SDCards are limited to 4 bits by the physical pin count. Taking a gamble I created a board to test this new eMMC chip. I created a fake SDCard!

This fake card allowed me to check very quickly if my assumptions were correct both at a hardware and a firmware level. I wanted to be sure that it was possible to interface the eMMC via 4 bits, rather than the full 8 bits and be sure the firmware instructions were the same between these two technologies.

The Atmel SAM4S chip has a hardware interface for SDCards. If the eMMC worked with the SDCard interface, I just needed to hook up the eMMC to the Atmel SAM4S chip using the built-in HSMCI interface. I could then use the libraries provided in the Atmel Software Framework to interface to the card. No effort required and definitely no flash translation layer required!

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