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Old 01-21-2016, 12:46 PM   #1
krishnakumar.r93
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Default I2C signal distance and signal booster

I am using MLX90614, which is a i2c output. My doubt is that, how long can i transmit the signal without any booster? I dont want to waste time in TRIAL and ERROR. My sensor is 30mts far from my MCU. Can the I2C signal reach the MCU with just the pullup resistor at the SCL and SDA pins.?
If no, please suggest me some booster ICs or SMBus ICs for taking the i2c signals faraway.
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Old 01-22-2016, 07:25 AM   #2
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The I2C bus specification no mention of the length of the cable, it is understood that this is an internal system bus. However, the standard imposes a limit on the pull-up resistor, and maximum capacity - 400 pF.

Somewhere I've read on the forums: I2C robust work on length of 50 m with copper cable STP.

You can try to use twisted pair CAT5, CAT6
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Old 01-22-2016, 10:09 AM   #3
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Best way to avoid trial and error is to reconsider the design, e.g. place a ┬ÁC near the sensor and use reliable RS485 communication over the distance. Or plan I2C operation at reduced speed of a few 10 kHz maximum.
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Old 01-22-2016, 10:16 AM   #4
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That would increase the cost. I'm trying to work it out on a lower budget. Is there any I2C booster, that you would recommend, or at a cheaper rate.?
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Old 01-22-2016, 10:47 AM   #5
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Hi,

Read I2C specifications.
This interface is developed for inter IC communication. Mainly for ICc on one board or at least in one box.

Nowadays this is exteded (somehow misused) to communicat between two electonic equippments.
Like the SMBus between PC and monitor

With the I2C bus and it's high impedance you may expect a great influence of environmental noise.
Also the single ended signalling without taking care of line impedance will generate overshoots and reflection on long distance.
Usually you need slow clock frequency for longer distance, but this also means you either need analog filters or slow receivers to avoid ringing at the receiver output.
Driving long lines meabs driving high capacitance. But as you now the the outputs are low ohmic open collectors, but the pullups are high impedance. So you don't have unique driving capabilities.

Next is the unidircectional (mostely, because some ICs use clock pulse extension) clock signal and the bidirectional data signal brings additional difficulties.
This especially is true when you want to install a booster. A booster for a bidirectional open collector ind has it's drawbacks. And how do you want to install them? Every 5 meters a boioster?

My recommendation: It is not even wort a try to use the I2C with a dustance of 30m.
For sure you may bring it to work at home or in laboratory condutions, but it is reliable to work in every customer's (electrical) environment.
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