I'm affraid I can't use SPI, tell me I'm wrong!

Projects General Guidance
1

Hello :slight_smile:

So I have the Arduino MEGA2560 with a TFT display and it's adapter shield to connect easily the display to the Arduino. On the TFT there is also a SD slot.

Here are the pins used by the adapter shield on the 36 pins connector of the Arduino:

You can see, the SD slot use the pin 53 for SD_CS.

I was randomly reading things on the Arduino main site, and on this page I can read:

On the Mega, the hardware SS pin, 53, is not used but it must be kept as an output or the SPI interface won't work.

I don't care about the Wifi shield as I don't plan to use it, but I don't understand if this line is about just the Wifi shield, or if it's a general information?
Ok I've read on the SPI page:

On the Arduino Mega, this is 50 (MISO), 51 (MOSI), 52 (SCK), and 53 (SS). MISO, MOSI, and SCK are also available in a consistent physical location on the ICSP header; this is useful, for example, in designing a shield that works on the Uno and the Mega.
Note that even if you're not using the SS pin, it must remain set as an output; otherwise, the SPI interface can be put into slave mode, rendering the library inoperative. It is, however, possible to use a pin other than pin 10 (53 in my case?) as the slave select (SS) pin. For example, the Arduino Ethernet shield uses pin 4 to control the SPI connection to the on-board SD card, and pin 10 to control the connection to the Ethernet controller.

But I don't really understand what it said... My SD slot use those 4 pins!

Because I plan to use an DS3234 RTC breakboard, and sensors, which will maybe use SPI, that's why I'm affraid of not being able to use them!

So... Will I have problems using the SD slot + the RTC + those sensors?

Thanks in advance!

2

Each SPI device needs its own Slave Select line.
If the SD slot uses 53, then your other devices will have to connect to different pins.

3

SS is for the slave to notify the master of its intent to transmit. It is the opposite of CS.

I am not sure why a SD card needs SS.

4

SS is for the slave to notify the master of its intent to transmit. It is the opposite of CS.

I am not sure why a SD card needs SS.

Wrong. SS is driven by master, tells slave to clock in data from master, and to clock out slave data.
SS and CS are used interchangeably.

5

Ok, I am reassured that I can choose whatever pin I want for the other SPI devices :slight_smile: But, in the schematic, does that mean that the SD slot is using SPI?

Now to find a solution to connect those other devices to the same pins 50, 51, 52.. Well I'm far from here anyway, but I like to anticipate things...

Thanks!

6

Are the other devices shields? Mega size or Uno size? External devices?

I just came up with a Mega Screw Shield, will easily let you take wires from the Mega to external devices.

Pics at Reply #54 here

Have a couple of bare boards available, $13.60 mailed to US locations. If you scroll back up the thread the headers & screw terminals used are listed.

7

Devices would be external, but I'm not sure to use SPI I would prefer I2C to be honest, less troubles with cables, but maybe I won't have the choice and your mega screw shield will be a good solution then :slight_smile: How much with shipping to France?

8

France - I don't know, need to check with my wife. She does the packing & shipping & post office runs. There's rules about size and also on flexibility. The card fits in a business envelope, and might be short enough to pass the flexibility requirement. Will let you know tomorrow.

9

guix,
I'm putting an order together here
http://arduino.cc/forum/index.php/topic,118807.60.html
please jump in and advise if you want a complete kit with headers and screw terminals, or if you will find your own parts and just nee a bare board.

Thanks
Crossroads