Electronic – Arduino SPI limit

arduinospi

Is there a limit to how many slaves you can control with a Arduino board using the SPI bus as long as you do not run out of pins?

Uno, for example, has 14 digital I/Os. So theoretically, I could have 11 slaves if I wanted to since MISO, MOSI and SCLK will be shared and there would be 11 different CS pins. I was wondering if anyone would run into any practical problems long before controlling 11 slaves.
( Although I will not use MISO )

I plan on controlling six 12 bit DACs for a project using UNO using the SPI bus. Is there something I need to be aware of?

Best Answer

The number of SPI slaves is not limited. In fact, you can add digital muxes (multiplexors) and control more slave devices than you have digital pins on the Arduino.

However, the SPI was design for communication over short distances within a box. So, the physical size of the bus can become a problem (bus capacitance, EMI). If you have to make a long-distance ruggedized SPI, there are application notes on the subject: Extending the SPI bus for long-distance communication.

Related Solutions

Electronic – arduino – Making two Arduinos talk over SPI

I took a quick look at the Arduino SPI library and couldn't find the attachInterrupt() function. I assume that you're using an extended library of some sort and I'll make the assumption that it works although I would like to see how it's implemented underneath.

A couple things that I saw:

This compiles? if (pos < sizeof buf) - it probably needs to be if (pos < sizeof(buf))
In the sample code on the forum, he has a key line right here that you're missing in your slave code:
```
// turn on interrupts
SPCR |= _BV(SPIE);
```

Without this your receive interrupt probably won't fire. I also don't see where global interrupts are being enabled although I have no idea if Arduino takes care of that for you under the hood. I also haven't bothered to look up what any of those registers mean, I'm trying my best to go by memory of AVR's SPI registers.

Electronic – arduino – Use unused SPI MISO pin for I/O on ATTiny

The SPI MISO function is nothing but a digital input pin.

The question does not specify whether the pseudo-SPI LED peripheral provides a handshake or other data on the MISO line at all. Assuming there is some initial data at set-up if at all, but no further use of the line by the peripheral during normal operation, the following should work:

Connect one of your 3 data source lines, OR'ed with the MOSI coming from the SPI peripheral, into pin 6 of the ATTiny.

During SPI set-up, ignore (or do what's needed with) the signal coming into pin 6. After that, simply read Pin 6 as you would any normal digital input pin.

If the LED peripheral works fine without connecting it to the MISO on the ATTiny at all, then you can skip the OR connection, and simply treat pin 6 as a standard digital input.

Best Answer

Related Solutions

Electronic – arduino – Making two Arduinos talk over SPI

Electronic – arduino – Use unused SPI MISO pin for I/O on ATTiny

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