I2C: How to connect multiple devices.

[JacekQ]

Hello,
I have another questions:
1. "Hotplug" - is it safe to connect/disconnect I2C device to working RPi?
If it's safe and should be like this:
- Connecting:

• GND
• 5V/3V3
• SCL
• SDA

- Disconnecting:

• SDA
• SCL
• 5V/3V3
• GND

2. Connecting multiple devices ...
I have:

• Sonar (5V)
• Servo driver (3V3)
• LCD + keypad (in the diagram the power supply is 5V but the plate is additionally connected 3V3)
• Gyroscope (2V5 to 5V5)

Can I connect these devices as shown in the attached diagram?

RPi I2C.png (22.29 KiB) Viewed 3501 times

[techpaul]

As long as

• Combined current from ALL devices is less than 50mA
• Combined 5V current from all devices is less than 200mA (maybe less depending on USB loading)
• NO devices have any pullups on their SDA and SCL - I2C lines
• All devices can properly handle 3V3 level I2C signals.

Be wary of hotplugging as this puts surges in power rails and can cause dips in power on Pi, maybe reseting it. Put at least a 10uF capacitor at Pi end for 5V and 3V3, to help alleviate this.

[JacekQ]

Hello,
I checked every device and only LCD requires 5V power supply. All other devices work with 3V3 or 5V. I think that better solution is to connect all devices to 5V but I found infromation than i2c devices should be powered 3V3 and if I want to use 5v then I have to use logic level shifter.
Do I have to use logic level shifter or will it work with 5V power supply without level shifter?

[techpaul]

Hello,
I checked every device and only LCD requires 5V power supply. All other devices work with 3V3 or 5V. I think that better solution is to connect all devices to 5V but I found infromation than i2c devices should be powered 3V3 and if I want to use 5v then I have to use logic level shifter.
Do I have to use logic level shifter or will it work with 5V power supply without level shifter?

In most cases and proper I2C interfaces, ensure all modules have NO pullup resistors on SDA and SCL, then if modules powered from 5V the I2C will work at 3V3. Only checking datasheets for each device can tell if the Voltage high threshold is low enough that when powered by 5V it will recognise anything over 3V as a high input.

[pjac]

problem solved.

is it possible, that the fact that I didn't solder the pins in GY-80 are causing so much problems? Is it necessary, or can I just plug it in?

apparently it is necessary...

Yes - I found that too, until I soldered the pins on the GY-80 board I couldn't detect the sensors. Simply slotting the pins in place was not enough.

P.S. I also found just four jumper wires was enough, blog post: http://astrobeano.blogspot.com/2014/01/ ... ry-pi.html

[Richard-TX]

Pull up resistors are already on the Pi board.

Just wire all the i2c devices in parallel, so connect all the SDA pins together, and all the SCL pins together. as shown here http://en.wikipedia.org/wiki/I%C2%B2C

Each one has it's own address so they don't interfere with each other.

PeterO

Just because the RPI has pullups does not mean that the remote device should not have them. Having 10k pullups will help balance the impedance at both ends of the wire. If the I2c Bus wires are very short ( < 1 meter) then pullups may be optional depending on bus speed. Try a i2cdetect at 1 mhz with and without pullups for an experiment.