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Garbage Characters on RGB_LCD Shield
Moderators: adafruit_support_bill, adafruit

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Re: Garbage Characters on RGB_LCD Shield

by 3nginerd on Thu Jun 25, 2020 1:55 pm

adafruit_support_bill wrote:Can you post some overall photos showing the rest of your wiring and connections?


Of course!

Please see below. Is it expected for I2C to have problems when trying to read/write to a device that isn't actually on the bus?

Image

Image

Image

Image

Image

FYI, the red box is not currently connected to the Arduino; it is what is controlled by the "Claw" tab referenced earlier. It communicates via Serial1.

Also, the loop of wires on each Tic is my temporary measure to enable motor movement; they are programmed to have fail safe limit switches, so that they stop movement if those lines are not connected, or a normally closed switch is pressed.

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Re: Garbage Characters on RGB_LCD Shield

by adafruit_support_bill on Thu Jun 25, 2020 2:21 pm

I see a lot of i2c bus length there. Looks like a couple of branches extending a foot or more. i2c is a short-distance protocol - originally intended for communication between chips on the same board. The maximum bus length is limited in part by the total bulk cable capacitance of all branches of the bus wiring. When you get much beyond 10-12" of total bus length, you can start running into problems with excessive signal rise-time.

The other limitation is interference. Again, longer wires make better antennae for electrical noise. Using twisted pair wiring often helps in that regard.

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Re: Garbage Characters on RGB_LCD Shield

by 3nginerd on Thu Jun 25, 2020 3:46 pm

adafruit_support_bill wrote:I see a lot of i2c bus length there. Looks like a couple of branches extending a foot or more.


You're quite right; it's the 3" jumper + 12" servo cable for a total of 15" from arduino to each Tic. Sorry if that wasn't clear before.

adafruit_support_bill wrote:i2c is a short-distance protocol - originally intended for communication between chips on the same board. The maximum bus length is limited in part by the total bulk cable capacitance of all branches of the bus wiring. When you get much beyond 10-12" of total bus length, you can start running into problems with excessive signal rise-time.

The other limitation is interference. Again, longer wires make better antennae for electrical noise. Using twisted pair wiring often helps in that regard.

I was aware of I2C limitations from previous projects, but I'm still confused as to why the LCD shield thinks the select button is being pressed when I have nothing connected besides the shield to the Arduino. Why is that happening? Should I troubleshoot the problems with the other devices on the bus first? or without them?

I've also shortened the cables to a total of 9" and the freezing persists.

Also, the Tics can be run by Serial as well, but I thought that would be more troublesome than devices which are explicitly listed as "I2C; works with other I2C devices". If Serial is the better answer though, so I can use the LCD shield properly, I'm okay with making the switch.

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Re: Garbage Characters on RGB_LCD Shield

by adafruit_support_bill on Thu Jun 25, 2020 4:33 pm

for a total of 15" from arduino to each Tic.

The distance between devices is not as important as the total length of all bus segments. 3" + 12" + 12" = 27".

The symptoms you are seeing are characteristics of bus problems. The fact that the problem goes away when you add devices seems to hint that the problem may be on the board itself. Can you post a photo of the bottom of the shield?

Also, take some resistance measurements from SDA to 5v and SCL to 5v.

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Re: Garbage Characters on RGB_LCD Shield

by 3nginerd on Thu Jun 25, 2020 7:13 pm

adafruit_support_bill wrote:The distance between devices is not as important as the total length of all bus segments. 3" + 12" + 12" = 27".

Sorry! Thanks for clearing up my misunderstanding!
adafruit_support_bill wrote:The symptoms you are seeing are characteristics of bus problems. The fact that the problem goes away when you add devices seems to hint that the problem may be on the board itself. Can you post a photo of the bottom of the shield?


Here you go, sorry I didn't get a good full-view picture of the back before:

Image

adafruit_support_bill wrote:Also, take some resistance measurements from SDA to 5v

4.62 kOhms
adafruit_support_bill wrote:and SCL to 5v.

4.66 kOhms

Thanks for all of your time, Bill!

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Re: Garbage Characters on RGB_LCD Shield

by adafruit_support_bill on Thu Jun 25, 2020 7:54 pm

No visible issues with the soldering. And the pullup values are within spec. I suspect that the issue is just a flaky MCP2307 port expander chip.

Did you purchase the shield directly from us, or from one of our distributors?

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Re: Garbage Characters on RGB_LCD Shield

by 3nginerd on Thu Jun 25, 2020 10:21 pm

adafruit_support_bill wrote:No visible issues with the soldering. And the pullup values are within spec. I suspect that the issue is just a flaky MCP2307 port expander chip.

Did you purchase the shield directly from us, or from one of our distributors?


Directly, Order # 2303312-5284096160. Thanks for all the help troubleshooting!

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Re: Garbage Characters on RGB_LCD Shield

by adafruit_support_bill on Fri Jun 26, 2020 5:39 am

Please contact support@adafruit.com with a link to this thread & we can replace the kit.

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Re: Garbage Characters on RGB_LCD Shield

by 3nginerd on Mon Jun 29, 2020 9:01 pm

Done, and on the way; thanks, Bill!

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