Implement SDIO to interface SPI device

2
votes

People,

I have always seen references about how to use a SPI interface to operate a SD memory card. This is not what I want. I need to do exactly the opposite.

I want to be able to use the SDIO controller (through SD slot) in my "host" (any PC having a SD-card interface) to talk to my devices (basically microcontrollers) that can only "speak" SPI.

If my understanding is not too wrong, I cannot simply tell my SD controller to talk in a raw SPI mode but I can teach my microcontrollers to behave as a SDIO device that can be controlled by my host.

This way I still have two challenges left:

  1. Correctly implement a generic SDIO device in my microcontroller.
  2. Implement/configure the correct drivers in the host to be able to interact with my devices.

Implement the SDIO device seems to be a matter of following the spec. The host-side driver, though, is something I hope I can accomplish with a user-space driver in Linux using some already existing kernel-space driver to SDIO.

That's the point that I come to ask for help. Can anyone please point me any samples, documents or any kind of resources that can help me in my task?

2
linuxlinux-device-drivermicrocontrollersd-cardspi
I doubt there are many people around here who understand how the SDIO stack works in Linux (and a driver using SDIO should behave). I'd definitely look them up and ask them directly, or via their mailing list. As a starting point: sdio-linux.sourceforge.net - Laszlo Valko

2 Answers

1
votes

On the PC side, this is all you need: http://sourceforge.net/projects/sdio-linux/

This may be useful for reference: http://www.varsanofiev.com/inside/WritingLinuxSDIODrivers.htm (although, I don't think you would be writing a driver)

On the microcontroller side, use "bit-banging" to implement the SDIO spec.

However, first consider why do this. SDIO and SPI are just serial protocols, so is USB; wouldn't you rather make an SPI-to-USB bridge? USB is much more user-friendly on the host side, as well as being more standard/more common. And if you do want a SPI-to-USB bridge, turns out it already exists, the SPI Shortcut (probably other options, this is just the first one that comes to mind)

EDIT Or, you could bit-bang I2C on the micro, if the host supports I2C (many do). Actually, go through every serial protocol the host supports, and see if you can support it easily from the micro side (by bit banging, since the micro is likely to not have a slave mode for that protocol built-in). RS232 (with level shifter), I2C, and SPI are likely to be the preferred choices. SDIO is pretty much the last choice, I think.

0
votes

SDIO is very tightly specified. Unless your microcontroller has an SDIO block that is designed to act as a device rather than host, I don't think this will be possible. I know of a few special purpose communications controllers that implement an SDIO device, but I haven't come across any general purpose microcontrollers.

You would need a fairly fast microcontroller to be able to bit-bang SDIO initialization at up to 400 kHz. If running an STM32F4 at 180 MHz, this gives you only microcontroller cycles between SDIO clock cycles. If the host turns up the clock speed to the maximum of 25 MHz after initialization, then you're down to 7 cycles between SDIO clocks.

For perspective on the SDIO spec, the one you linked is a simplified spec that doesn't cover the signalling and timing of the bus. The full spec is many times larger.

As Alex I mentioned, there may be better alternatives for what you need. If your SDIO host supports SPI mode, most microcontrollers do have SPI peripherals that can act as slaves rather than hosts, so this may be an avenue without a peripheral. If your data rates are low enough, a simple UART may suffice (you can reasonably hit 1 Mbit over short distances).