Hello everyone.
I am developing a custom SDIO card for interfacing a PDA to a microcontroller in a research project. I got the SDIO docs and I have implemented some physical layer code that allows me to receive commands sent by the SDIO host (PDA) and to send the necessary response. I am stuck in the initialization phase of the card, after the SDIO host sends the CMD7 to address the card. I send the response to the host, but then the host sends a CMD20, which the documentation says is reserved to SD memory cards! Of course, the OCR describes a card with 1 I/O functions and 0 memory functions. I am using one of the SPI ports in the microcontroller wired to the CLK and CMD lines to send and receive data. I am emulating 1-bit SDIO mode, *NOT* SPI.
The sequence of commands I receive from the SDIO host is: CMD5 CMD5 CMD3 CMD7 CMD20