Confused on SPI.. Any help??

Projects Networking, Protocols, and Devices
1

Guys,
I'm basically developing a hardware from bare metal... I mean this is not a Arduino shield or boards, but the hardware will be custom designed. The architecture is kind of arduino, actually from the point of view of software. I will use the IDE of Arduino.

Now, At this stage, I'm totally confused on the Arduino SS pin for SPI. Let me come to straight to the point:

  1. Hardware will be based on mega2560.

  2. It will have SD card and Ethernet both and both will be used via the SPI.

From my understanding, I found these:

  1. SD SS pin is D4 for shield.
  2. Eth SS pin is D10 for shield.
  3. D53 in mega2560 is the hardware SS

After reading and searching I found many info and really confused. Now I'm considering my understanding as follows:

  1. Always keep D53 to OUTPUT mode for SPI.
  2. When I will need to use the SD card, I need to Put the pin to OUTPUT & LOW of the card's CS pin (and as per the rule of thumb due to library, I can just use the D4)
  3. When I will need to use the Ethernet (Wiznet5100/5200), I can do the same, the D10 pin OUTPUT and LOW for the pin of Eth device's CS.

[Question 1] Now, as the device will work for both SD and Eth almost the same time (yeah, not simultaneous), then when i have to use the SD card, I need to first make the D10 (for eth disable) to HIGH. Again when I need to use the Eth, the opposite, D4 to HIGH.

[Question 2] If I'm not wrong, then do I need to make it LOW again once the job is done?? [Coz i found in a thread someone made a D10 high for SD init and then directly did the ethernet init...]

[Question 3] So as it will be a device that will be in a server mode (listening for connections) hence I can just keep the D10 LOW and when only and when it will need to write something in SD, I can temporarily make the D10 HIGH for eth disable and SD enable and then later I can just shift it back again just to keep ethernet continuously running right?

Thanks in advance!!
Mishu~

2

Every SPI device shares the three lines MOSI, MISO and SCK.

And each device must have its own chip-select pin on the Arduino. You do not have to use the Arduino SS pin for chip-select. Any Arduino I/O pin can be used.

However, even if it is not used for chip-select, it is essential to have the Arduino SS pin set at OUTPUT in order to ensure that the Arduino acts as SPI master.

...R

3

hi, your above reply gives the ans of Q1 and !2 so far I can understand... BBut what about the Q3?

4
  1. Yes, but not really.

The process of setting the CS line to LOW is a signal to the slave device that communication is about to start. So usually the line is left HIGH all the time unless you have something to say.

I haven't done much with Arduino and Ethernet but I'm pretty sure that the Ethernet library will manage the CS line by itself and you don't need to worry about it.

5

MorganS:
Ethernet library will manage the CS line by itself and you don't need to worry about it.

Does this mean that the same applies for SD card too??? I mean then after initialize in setup, i just can use whenever and whereever i need too use the regular functions???

6

Yes. That's why we use libraries and standard protocols like SPI. You don't have to re-invent everything for each new sketch.

7

great!! Then i think problem is solved!!