I2C over 5m cable but low power - Page 1
Forum section: electronics / projects

author: veryevil
author_rank: Unknown
timestamp: September 21, 2018, 10:41:46 am
content: Hi, I'm looking at having two I2C slave sensors located up to 5m away from the I2C master and was looking for a low power way to do this.There is the P82B715 but it uses a lot of current and going differential with PCA9615 seems even worse with the bias resistors burning current.I was hoping to keep it to 4 wires, VCC, GND, SCL and SDA but could go to 6 for differential.I assume twisted pair is the way to go with differential and I've read that I2C might be able to do it without any other IC if a constant current is used instead of pull up resistors and using twised pair with VCC and SCL twisted and GND and SDA twisted?Any suggestions?Thanks

author: Rerouter
author_rank: Super Contributor
timestamp: September 21, 2018, 10:49:09 am
content: Differential is a little painful for I2C,I have successfully done 3m runs of 400KHz, using 2 runs of shielded twin core instrumentation cable, 1 being SCL and Ground, the other being SDA and VCC,The sheilds where floating, it was just to keep down crosstalk down between SDA and SCL,

author: OM222O
author_rank: Frequent Contributor
timestamp: September 21, 2018, 10:50:58 am
content: What do you mean by low power? Miliwatts? Micro watts? What power source are you using?I think the simplest way would be to boost the voltage and use high value resistors so there's no current. Something like 12 or 24 volts would be perfectly acceptable, or you can go with ethernet (48 volts). You just need a voltage comparator on the recieving end to convert that range of voltage to a logic level signal. You can switch on and off the high voltage signals simply by using two BJTs and feeding SDA/SCL to the base of of the transistor. Basically a very crude voltage shifter.

author: veryevil
author_rank: Unknown
timestamp: September 21, 2018, 11:03:09 am
content: Hi, the plan is to use a lithium primary battery at 3.6V but I may have a boost up to 5v on board. End sensors can run on 5v too.

author: ogden
author_rank: Super Contributor
timestamp: September 21, 2018, 12:16:14 pm
content: Quote from: veryevil on September 21, 2018, 10:41:46 amNo, you can't convert i2c to differential w/o IC's. For 5m you don't need differential signals. Use one pair for SCL+GND, cecond pair for SDA+GND, third pair for VCC and fourth for GND. To lower consumption, you canlowerincrease pull-up resistance. 4.7KOhm with some capacitor could work, for slow clock speeds. On SCL and SDA you must add noise filter capacitors. Calculate pull-up + capacitor RC time constant to be at least 1/8 of i2c clock period. Other way - add something like 330pF, then check signals on the scope, compare to i2c datasheet chapter about i2c waveforms & signal integrity, change capacitor or clock speed if necessary:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 21, 2018, 12:24:19 pm
content: How fast? Heavy RC filtering is going to be the recommended course here, for dealing with noise. The sensor itself can be powered down or disabled (if it won't load down the bus in the process, or if the bus is dedicated to the sensor so it doesn't matter). That's the easiest way to solve it, without having to add extra interfaces.Tim

author: veryevil
author_rank: Unknown
timestamp: September 21, 2018, 12:33:39 pm
content: Hi, thank for the suggestions. Worried a little bit about immunity but 5m isn't too long. Only after 400khz or 100khz would be alright at a pushJust seen the maximum 1 wire solutions ds2482 and ds28e17 which looks very low power. A bit expensive but might be a good trade off.Will have to investigate

author: ogden
author_rank: Super Contributor
timestamp: September 21, 2018, 12:45:50 pm
content: Quote from: veryevil on September 21, 2018, 12:33:39 pmYou shall aim at 10KHz or so. 1-wire is not that much lower power than properly designed i2c sensor. As mentioned, for low power you shall power-down sensor together with i2c pull-ups. Note - if you don't "power-down" pull-ups, your sensor i2c IC most likely will draw power through them.

author: jbb
author_rank: Super Contributor
timestamp: September 26, 2018, 09:28:40 am
content: It's not very low power (some mA, I believe), but Analog (Linear Tech) have the brand-spanking-new LTC4331 that might be appropriate.They seem to have a few differential-over-CAT5 chips coming out.