0

LTC4311 I2C Extender
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

LTC4311 I2C Extender

by Stef94 on Thu Jul 29, 2021 1:25 pm

Hello,

I have the I2C extender LTC4311. Now as I have seen, the EN pin is connected to VIN with a resistor of about 10k. For usage of the extender I have only to connect it to SCL, SDA, GND and 5V of my Arduino. I can solder directly two wires per pin for connecting it between the Arduino and an I2C device. Is that correct?

For what usage is made a solder pin for EN on this board?

Next question is about the resistors for SCL/SDA. Do I need an additional resistor for my Arduino Mega? The Mega has a 10k on each line, the extender has also 10k on each line, that means it is a resistance of about 5k on SDA amd 5k on SCL. Should I add a 4.7k to the lines anyway as normally done or is not needed? My wires are around 1.50m long.

Best regards

Stef
Last edited by Stef94 on Thu Jul 29, 2021 1:46 pm, edited 1 time in total.

Stef94
 
Posts: 15
Joined: Mon Jun 08, 2020 10:38 am

Re: LTC4311 I2C Extender

by adafruit_support_bill on Thu Jul 29, 2021 1:42 pm

Yes, you can connect 2 wires each to the SDA & SCL. Pulling the EN pin low disables the active termination, so your i2c bus will go back to its normal behavior with passive pullups.

Not sure what use-case the chip designers had in mind for the EN. But the pin is there on the chip, so we added a breakout for it.

adafruit_support_bill
 
Posts: 81927
Joined: Sat Feb 07, 2009 10:11 am

Re: LTC4311 I2C Extender

by Stef94 on Thu Jul 29, 2021 2:51 pm

Okay, that is great. If the EN pin is pulled low it is out of the circuit, but then I can also remove it completely.

I have edited the first post, but think you have not seen it. I will post it gere again.

Next question is about the resistors for SCL/SDA. Do I need an additional resistor for my Arduino Mega? The Mega has a 10k on each line, the extender has also 10k on each line, that means it is a resistance of about 5k on SDA amd 5k on SCL. Should I add a 4.7k to the lines anyway as normally done or is not needed? My wires are around 1.50m long.

Stef94
 
Posts: 15
Joined: Mon Jun 08, 2020 10:38 am

Re: LTC4311 I2C Extender

by adafruit_support_bill on Thu Jul 29, 2021 3:33 pm

You should not need any additional pullups. 5K in parallel resistance is more than strong enough. Too many pullups on the bus make it harder for devices to drive it low when needed.

adafruit_support_bill
 
Posts: 81927
Joined: Sat Feb 07, 2009 10:11 am

Re: LTC4311 I2C Extender

by Stef94 on Thu Jul 29, 2021 6:47 pm

The Arduino Mega has 10k built in already. The I2C extender has too 10k built in. I could add 4.7k in addition, this way it lowers the resistance to 2.4k in total. Will this be better? The number of resistors is not a problem I think, it is the value that should not be too high, right? But what I have read, most of the users from an Arduino Uno that has no resistors built in and need some externally resistors, they have used 4.7k.

Stef94
 
Posts: 15
Joined: Mon Jun 08, 2020 10:38 am

Re: LTC4311 I2C Extender

by adafruit_support_bill on Thu Jul 29, 2021 7:17 pm

No. Lower resistance is is not necessarily better. As I said in my previous post, too many pullups on the bus make it harder for devices to drive it low when needed.

4.7K is sort of a loose 'rule of thumb' for the total parallel resistance. The 'perfect' value varies a bit depending on the length of the bus, the number of devices and the logic levels being used. But for most small-scale applications anything in the 2.5K-10K range works.

All of our i2c devices are built with 10K pullups. That typically allows you to get about 4 devices on the bus before excess pullup strength starts to cause problems. If you have many devices on the bus, you may need to remove pullups from some of them to keep the total parallel resistance to a workable level.

adafruit_support_bill
 
Posts: 81927
Joined: Sat Feb 07, 2009 10:11 am

Re: LTC4311 I2C Extender

by Stef94 on Fri Jul 30, 2021 11:37 am

I have tried it today to hook up to my Arduino Mega. After hooking up, the Mega sends nothing over I2C.
I have connected it to the pin 20 and 21 for SDA and SCL with a double connector. On one end it is the extender, the other has the 3 devices. What could be wrong? 5V and GND is connected of course too.

Stef94
 
Posts: 15
Joined: Mon Jun 08, 2020 10:38 am

Re: LTC4311 I2C Extender

by adafruit_support_bill on Fri Jul 30, 2021 11:39 am

Please post some photos showing all your soldering and connections.

adafruit_support_bill
 
Posts: 81927
Joined: Sat Feb 07, 2009 10:11 am

Re: LTC4311 I2C Extender

by Stef94 on Fri Jul 30, 2021 12:16 pm

The solder joints on the PCB are temporary only. They are not very good, but if I tire on each wire separately, they are holding.

PCB.jpg
PCB.jpg (353.8 KiB) Viewed 67 times


The two 4.7k resistors on my PCB are connected to SDA/SCL where the solder joints are, the other end goes to 5V. The other three Arduinos that are connected to I2C are put on the PCB, the connector on top of the picture, these pins are also connected to SDA/SCL where I have soldered the wires for the I2C extender.

I2C_Extender.jpg
I2C_Extender.jpg (457.89 KiB) Viewed 67 times

Stef94
 
Posts: 15
Joined: Mon Jun 08, 2020 10:38 am

Re: LTC4311 I2C Extender

by adafruit_support_bill on Fri Jul 30, 2021 12:44 pm

Probably too many pullups on the bus. Please re-read my previous responses.

adafruit_support_bill
 
Posts: 81927
Joined: Sat Feb 07, 2009 10:11 am

Re: LTC4311 I2C Extender

by Stef94 on Fri Jul 30, 2021 12:54 pm

Yes, I remember! The only resistors I can remove are the 4.7k on my PCB. The Mega has some 10k built in and on the I2C extender are also 10k resistors built in that can not be removed easily. I will try it to remove my 4.7k resistors.

Stef94
 
Posts: 15
Joined: Mon Jun 08, 2020 10:38 am

Re: LTC4311 I2C Extender

by Stef94 on Fri Jul 30, 2021 1:08 pm

Tried to desolder the 4.7k resistors without succes! The I2C connection does not work!
Is it a problem, if I connect the 5V and GND to the other Arduino and not the Mega? All Arduinos that are connected over I2C have the same 5V and GND source from a battery. For that reason I think it is not a problem?

Stef94
 
Posts: 15
Joined: Mon Jun 08, 2020 10:38 am

Re: LTC4311 I2C Extender

by adafruit_support_bill on Fri Jul 30, 2021 1:12 pm

All signals are measured relative to ground. If the Mega is on the i2c bus, it need to be connected to the bus ground also.

adafruit_support_bill
 
Posts: 81927
Joined: Sat Feb 07, 2009 10:11 am

Re: LTC4311 I2C Extender

by Stef94 on Fri Jul 30, 2021 1:14 pm

adafruit_support_bill wrote:All signals are measured relative to ground. If the Mega is on the i2c bus, it need to be connected to the bus ground also.


Three Arduino Uno and one Mega is connected on my battery and share 5V and GND. The Mega is the master that sends the commands to the three Unos. All Arduinos are on the same I2C bus. The extender is connected to the Mega, but the 5V and GND are connected to an Uno because of the free pins on it. That is okay this way, correct?

Stef94
 
Posts: 15
Joined: Mon Jun 08, 2020 10:38 am

Re: LTC4311 I2C Extender

by Stef94 on Fri Jul 30, 2021 1:23 pm

I have removed the 5V wire from the I2C extender and now it works! The power LED is turned on anyway. Is that normal?
Should I connect only SCL, SDA and GND to the LTC4311 to get it working?

Stef94
 
Posts: 15
Joined: Mon Jun 08, 2020 10:38 am

Please be positive and constructive with your questions and comments.