i'm building a device with a microsd card on it for data logging. i need to read this card but don't want to remove it from my device. there are loads of cheap microsd to USB readers out there - but does anyone know of a chip that does this function that i can build into my device? if i search the net i just get all the card readers.
You mean like an arduino with a microSD shield that your PC could query for data?
Disassemble one of those inexpensive micro-SD card readers and see what chip they use. Either design-in that chip or wire the micro-SD reader to your circuit. Either way you will need to keep the Arduino off the SPI bus while the USB interface is using the chip. That should be as simple as pulling down the Reset pin while the USB reader is active.
Or you can view the scematic of the data logger shield:
There is an scematic of the shield, they use a 74AHC125N for the power conversion from 5V to 3.3V.
The parts of the real time clock you can forget if you don't need that function.
i probably didn't explain this very well. Obviously i could read the microSD card from the arduino and then use a serial link etc to get the data to the PC. But for various reasons this does not work for my application - the PC must see the my device as an external file system not as a serial port. So i think i'm best to use whatever chip goes into those card readers. There's millions of the damn things on sale, so i guess there must be an in-volume chip being produced to do this - i just can't find out what it is! Tried breaking open a cheap card reader but the chip was encased in a rock-hard cement and it was impossible to get at it.
A quick Google search for "SD USB reader chip" turned up the Maxim 14502 as the first hit:
It appears to do exactly what you want. It goes between the SD card and your processor and can be set to pass-though mode to give your processor direct access to the SD card. It also goes between your system and the USB connector so it can share the same USB wire. With a single output pin you can switch it between pass-though (SD card and USB connected to Arduino) and Card Reader (USB connected to SD card reader).
It might be a little hard to solder with since it's in a tiny 40-pad leadless surface-mount package.
Mouser has them in single quantities for $11.58.
perfect! thanks for finding this. i had searched google a few times but all i got was a bunch of card readers.