Adafruit_CharLCDPlate not displaying text on v2 pi

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wikenator
 
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Adafruit_CharLCDPlate not displaying text on v2 pi

Post by wikenator »

I originally had the Char LCD plate connected to a version 1 Pi and it worked perfectly. Just today I moved the LCD plate to a version 2 Pi, and the contrast pot will not display text well (if i turn the pot completely to the left, I get a very faint outline of text, nothing readable from a distance of more than 1 foot away; turning the pot any further to the right i just get a blank display (no square blocks)). Is there a way to 'reset' the contrast so that the adjustments actually get me some contrast on the display?

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adafruit_support_mike
 
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Re: Adafruit_CharLCDPlate not displaying text on v2 pi

Post by adafruit_support_mike »

That's very strange..

Does the plate still work when you connect it to the v1 RasPi?

wikenator
 
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Re: Adafruit_CharLCDPlate not displaying text on v2 pi

Post by wikenator »

Yes, the v1 Pi shows text clearly with the pot turned almost all the way to the left. I put the LCD plate back on the v2, and turning the pot completely to the left still shows text that is about the same color as the backlight.

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Re: Adafruit_CharLCDPlate not displaying text on v2 pi

Post by wikenator »

Here is some more info on the steps I took if it helps:

The lcd screen was connected using jumper wires to the gpio breakout kit (i selectively soldered headers to the 4 necessary locations on the lcd plate.) When intially connected to the v1, the pot was turned about halfway around. I remember tuning the contrast where solid blocks showed up when turning the pot to the left, and transparent characters showed up when turning the pot to the right (or the other way around, I don't quite remember). The lcd screen worked perfectly for over a year.

When I connected the lcd to the v2, I just moved the ribbon cable from the v1 to the v2. All existing connections remained the same between the lcd plate and the gpio breakout. I downloaded the git code for the charLCDplate fresh to the v2, and changed all necessary values (busnum = 1) in all of my scripts. I ran the test script from the Adafruit_CharLCDPlate folder, and that is when i noticed no characters on the screen. I was only able to receive a faint visual of the characters on the lcd screen when the pot was turned completely left, as mentioned before. The backlight turns on and off just fine.

When i connected the lcd back to the v1 (keeping the pot turned to the left from where i had it on the v2), i was able to see characters clearly. but this time, turning the pot to the right (not even before reaching the halfway point) makes the letters disappear again, where before the characters were clearest AT the halfway point.

Connecting back the v2, it was same as before: pot all the way to the left, faint letters.

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adafruit_support_mike
 
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Re: Adafruit_CharLCDPlate not displaying text on v2 pi

Post by adafruit_support_mike »

Hmm.. I'm torn between diagnoses.

The symptoms you're describing sound like the LCD works but the contrast pot isn't getting enough voltage to bias the screen correctly on the Rev2 RasPi. That would be extremely strange, since the contrast pot gets its power straight from the 5v pin on the RasPi's GPIO header, and that pin is the same for either revision of the RasPi.

There's a difference in the I2C channels from Rev1 to Rev2.. Rev1 uses channel 0, Rev1 uses channel 1. That's the most likely problem, but it wouldn't explain your seeing characters when the contrast is cranked up all the way.

Take a voltage measurement between the LCD's first and third pins, counting from the left when the row of pins is at the top of the LCD. If you want to verify the connections with the Plate unpowered, pin #1 should be GND and pin #3 should be connected to the contrast pot's wiper (the simglesingle pin closest to the LCD). IIRC, the contrast works best when pin #3 is around 1v.

wikenator
 
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Re: Adafruit_CharLCDPlate not displaying text on v2 pi

Post by wikenator »

So, I determined that the issue lied with the powered hub to which I have the RasPis connected. Based on some of the FAQs on the Raspberry Pi website, the hub I have does not supply enough current for the v2 Pi to run itself and a peripheral. Apparently the v1 Pi is less power hungry. Here are the readings with each Pi attached to the hub:

v1 pot range: 0 - 4.45 V
v2 pot range: 0 - 4.12 V

Plugging the Pis directly into the wall, I was finally getting well-contrasted text on the LCD screen when connected to the v2. However, I still had to keep the pot turned to ~0V. Moving the LCD back to the v1, I had to turn the pot voltage up a bit (~2V) to get the clearest text.

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adafruit_support_mike
 
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Re: Adafruit_CharLCDPlate not displaying text on v2 pi

Post by adafruit_support_mike »

Hmm.. I hadn't seen that specific problem before. Thank you for letting me know about it so I can look out for it in the future.

I'm glad to hear the LCD is working for you though. Happy hacking!

manojm
 
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Re: Adafruit_CharLCDPlate not displaying text on v2 pi

Post by manojm »

Hi,

I have RGB LCD 16x20 and observed strange problem display text. It's connected with MCP IC-MCP23017 and work fine with RPI B+ with tutorial code. But next day when I try only backlight chaning colour but no text display. Please advice. I also check soldering and connections seems fine.

Seeking your help.
Thank you,
Manoj

manojm
 
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Re: Adafruit_CharLCDPlate not displaying text on v2 pi

Post by manojm »

Hi All,

I found the issue related to some IC pin soldering removed slightly and not connecting properly. Once I re soldered the pins it's start working. This is very difficult to figure out in circuit boards. Breadboard it's easy.

Hope this helpful for beginners.

Thank you,
Manoj Mallawaarachchie.

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