How to Connect Arduino Uno to Multiple SPI Slaves (SD Shield, Display) Sharing MISO?

Hi Everyone!

I'm actually working on a project wherein a arduino uno is connected with multiple slave devices(SD Shield and Display Board) and uses the SPI interface. Upon the project, I stumble upon the fact that only one slave can use the SPI at a time. The MISO pin on the arduino seems to be the problem. Can anyone help me understand this?

Any suggestions on how to use both slave devices at the same time? I already tried different configurations on how to do multiple spi but I have no luck.

Thanks in advance!

In a multiple slave SPI arrangement, the MISO, MOSI, and SCLK are all shared between the master and all the slaves. Every slave can drive the MISO pin, so the way to determine which one can drive it at a particular time is by using the chip select signals. Each slave has its own chip select, and only one should be active (low) at any time. If you don't have individual chip select signals for each slave, or if you are driving more than one chip select low at the same time, that would explain contention/collision on the MISO line.

So you can't really use two slave devices on the same SPI bus at the same time. You have to take turns sending/receiving with one slave at a time. Most datasheets show an SPI timing diagram where the chip select goes low, then the send/receive happens, then the chip select goes high. Then you can move on to the next slave and do the same thing.

If the Arduino has two SPI buses, you can put one slave on each one and use them simultaneously.

As Abe said, you can't do what you want without separate Chip Selects. MISO is a tri-sate pin, it is controlled by the CS, if you have multiple SPI devices, and one CS, then they will all be trying to drive the MISO line, and bus contention is the result. I don't know anything about the Arduino and the shield boards, but I would have thought that if multiple SPI devices were being used on the Arduino, that they should have allowed for it, if not, then that is weakness of the arduino environment The common way is to allocate any available I/O pins for the CS, the hardware would have to allow the appropriate linking of those I/O pins to the various CS pins on the shield boards, and the Arduino SPI library should allow the SPI devices to be used in a non slave mode, whereby the SS line is not used as the CS, but rather the I/O pins you have selected for the CS lines. I commonly use multiple SPI devices ( up to five or six) on the AVR, the compiler library I use for SPI allows the SS mode to be turned off, and I then have to allocate I/O lines, and use a few extra lines of code to take the appropriate I/O line low, send the SPI data, and then take it high again.
If the sheilds allow CS to be allocated to I/O port pins, but the arduino software development platform does not, then you may have to write your own library, or use another compiler for the AVR which does.
Cheers,
Richard