I2C pins

Using Arduino Networking, Protocols, and Devices
1

Hi,

sorry for this stupid question:
when i want to use I2C with the arduino, which pins do i loose for that/which cant be used otherwise that moment?

Is it this?
"TWI: A4 or SDA pin and A5 or SCL pin. Support TWI communication using the Wire library. "

or "SPI: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK). These pins support SPI communication using the SPI library. "

or "Serial: 0 (RX) and 1 (TX). Used to receive (RX) and transmit (TX) TTL serial data. These pins are connected to the corresponding pins of the ATmega8U2 USB-to-TTL Serial chip. "

All that stuff that i dont understand really gets me confused :slight_smile:

2

ah ok,
i think id loose 2 analog pins

3

Yep, A4 and A5.

Rob

4

There are three forms of serial communication on the Arduino by standard.

  1. SPI - pins D11-D13.

This is a synchronous bi-directional full duplex 1:1 serial communication system. It uses one line for the synchronous clock (D13) and two lines for data in and data out (D11/12).

  1. I²C - Pins A4 & A5

This is a synchronous bi-directional half duplex 1:many bus based serial communication system. One line is used for the clock, and one line for the data. An address is included in the data to select a specific target device. The data and clock lines should be pulled high with a large value resistor.

  1. RS-232 - pins D0 & D1

This is an asynchronous bi-directional full duplex 1:1 serial communication system. It is what the Arduino uses for programming, and is the same system that has been available on PCs since the dawn of technology. This is typically much slower than SPI and I²C but can talk over much longer distances and to more external devices. Typically the signals from this system are passed through a line driver chip such as the MAX232 to convert the 0-5V TTL* signal to a +12V to -12V (or thereabouts) true RS-232 signal.

* Actually TTL is old and rarely used. It's actually HCMOS these days. Same voltage range, but different signal discipline.

5

thanks a lot majenko.

"1. SPI - pins D11-D13.

This is a synchronous bi-directional full duplex 1:1 serial communication system. It uses one line for the synchronous clock (D13) and two lines for data in and data out (D11/12)."

For what kind of stuff/devices is that usually used?

I understand that for I2C u can attach a lot of devices, but i wonder if different devices use different clocks or will they all work with the same one?

So i can put a I2C pressure sensor, temp sensor, LCD display and whats not all on A5 and A4 and it will work, provided i address them right and use the right code?

And could someone please tell me what breakout boards r for? i see them alot

Edit:
is a breakoutboard just a way to access the tiny connections on a chip like smd with larger cables?

6

For what kind of stuff/devices is that usually used?

Fast stuff, like the Ethernet shield.

I understand that for I2C u can attach a lot of devices, but i wonder if different devices use different clocks or will they all work with the same one?

I2C has several fixed clock speeds, but for this sort of device the two commonest ones are 100 and 400kHz.
Slow I2C devices may not work at 400kHz, but 400kHz devices will work at 100kHz (or even very much slower)

So i can put a I2C pressure sensor, temp sensor, LCD display and whats not all on A5 and A4 and it will work, provided i address them right and use the right code?

Assuming they've all got unique addresses, they should work.

7

SPI and I²C are used for similar devices. There are often 2 versions of the same device available.

I²C uses less pins, but is slower than SPI, so it's a speed/space tradeoff between the two.

And yes, you put multiple devices on I²C and, with the right addresses and code, you can get at them all.

A breakout board is just a way to access tiny connections. Typically they are for tiny chips like TSSOP, TQFP, DFN, QFN etc.

8

thanks a lot,

and what happens when i want to use 2 times the same LCD display with the same address 0x27? or how can i use them both?

9

and what happens when i want to use 2 times the same LCD display with the same address 0x27?

You can't.
The two devices must have different addresses.
You can bit-bang I2C, but that is pretty tedious.

10

2 times the same LCD display with the same address 0x27?

Normally you can't, however if you're desperate you can use a chip like the LTC4305, that splits the bus into two independent buses.

In this case you may be better using an SPI version of the device (if available), with SPI you can have as many devices (identical or otherwise) as you like but each one needs a pin to select it.

Rob