Noobie SPI Help..

Forum 2005-2010 (read only) Hardware Interfacing
1

I want to attempt to build a web based sensor host using my Arduino ..

I will be using the following:

  • enc28j60 (SPI)
  • rmf12 Radio (SPI)
  • sdcard (SPI)
  • ds1307 (i2c)

So what confuses the heck out of me is first off how would I go about hooking up 3 spi devices on the same arduino, and secondly how would I access them all at the same time.

Would I need to buffer all of these with something like a 74AHC125 so they all talk / play nice?

Any help / tutorial would be appreciated.. Its my first project in 20 years, so I am new to the platform and forget most of my electronics training..

Abysmal :slight_smile:

2

SPI shares a common bus with MISO, MOSI, and SCK. The CS line has to be unique for each device.

You can't use them "at the same time", but you can use them all on the same bus, one at a time.

The ds1307 is no problem, as it lives on a different bus - the I2C bus.

-j

3

So from what I have read since posting my question and what you have said, if I had 3 SPI devices, I would need the 3 common pins ( MISO, MOSI, and SCK ) and then 3 unused pins, 1 each for the unique device select.

Now, you said they all can't be on at the same time. So does that mean that when I toggle 1 on I have to toggle the other 2 off each and every time? If so, is there an easy way of doing that?

Thank for the help..

Abysmal

Why is one first project always the hardest, and make one look the stupidest! :stuck_out_tongue:

4

The “chip” select is usually active low.

5

So what I need to do select a device is to set the CS to low on the device I want to use and all of the rest to High to deactivate them?

6

So what I need to do select a device is to set the CS to low on the device I want to use and all of the rest to High to deactivate them?

Correct.

Rob

7

CS is frequently, but not always, active low. For example, the DS1722 digital temperature sensor uses active high on CS. Check the datasheet for each peripheral and see.

Because they vary, and because active low logic sometimes confuses me, I like to do things like

#define MCP3208_SELECT 0
#define MCP3208_DESELECT 1

....

digitalWrite(pin, MCP3208_SELECT);
/* do SPI stuff */
digitalWrite(pin, MCP3208_DESELECT);

(cue the #define vs. const comments.... :slight_smile: )

-j

8

Thanks for all the help and advice..