Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I am trying to create a smart alarm system for my house. I want to have one arduino as the master and several other AtTiny's as slaves to collect data as temperature, moisture, gas concentration, opened doors etc.
How do I connect multiple wired AtTiny's to a master Arduino? Which is the most clever way to communicate between them ? I have read that AtTiny's do not support spi or i2c but you can "hack" it somehow.
EDIT: the distance between the master and the slave can be 1 to 10 meters.
If you happen to use those ATtiny that do not have hardware support of SPI or I2C, you can likely implement that in software, assuming you wont have any CPU-heavy processing running on the ATtiny (which is typically the case for mere sensor reading). That said, except for the 6-pins ones, most ATtiny's do have hardware support for both I2C and SPI (see product selector).
Choosing between I2C and SPI, I would go for the former given the multiple slave setup and the limited number of pins of the ATtiny (2 pins needed for I2C and 4 for SPI).
Actually, the ATTinys have hardware to support TWI (the I2C variant) or SPI, known as the "Universal Serial Interface." You can do one of them at a time, not the other. Also, you have to do protocol things like "look for the proper slave address" in software -- the hardware will detect the start condition for you and hold the clock low until you're ready to receive a byte, but you then have to compare that byte to your own address, and then enter a polling loop if it matches. (Note that the I2C rise/fall times are not guaranteed on these devices.)
Same thing for SPI; you can get interrupts and data for the SPI slave select and data transfer, and it can shift out data from you to the master as well in hardware, but you have to feed it each byte manually, and as a master, you actually have to generate the clock manually (entering a loop that strobes the clock bit.)
I have used I2C for medium-distance bus communications on a robot, and it worked fine. I don't know if it would work as-is over 10 Meters -- you might want to modulate onto a pair of RS-485 wires and de-modulate on the other end, for example. That would probably work very well, at 100 kHz or so. RS-485 driver circuits are cheap and easy to work with. You'd need two twisted pairs for the communications, so regular phone wiring. (More if you also want to send power.)
If that's too much bother for you, I'd suggest using XBees for comms with series 1 firmware (so, regular "air wires" for UARTs.) The Attinys don't do UART natively, but you can emulate it with the USI (single-duplex only) or bit banging (slow data rates only.)
That being said, why are you using ATTinys? The cost of a ATmega in quantity 25 is something like $2. You might as well have a real chip on each end :-)
