16x2 Blue LCD Kit for raspi

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brent_1
 
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16x2 Blue LCD Kit for raspi

Post by brent_1 »

After carefully assembling the LCD kit, then soldering the Blue 16x2 LCD to it, only the back-light works when connected to the Raspbarry Pi.

No characters are displayed.

Pushing the button next to Select lit the LED backlight, but no text was visible while running Adafruit_CharLCD.py.

There are no detectable shorts between adjacent pins.

After disconnecting and probing for continuity, and re-connecting to the rPi, the rPI won't boot. Something is seriously wrong, because if it is disconnected, the rPi LEDs immediately light and it boots.

How can this be debugged?

Could the MCP IO extender be bad?
Image

All the contacts marked with a green arrow are connected (< 1 Ohm between them).
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ImageImage
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brent_1
 
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Re: 16x2 Blue LCD Kit for raspi

Post by brent_1 »

After rebooting the rPi, it's back to connecting again:

sudo i2cdetect -y 1
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: 20 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

The PiPlate can be seen on the I2C bus (at addr 0x20), but only the LCD Backlight works.

1. cd Adafruit-Raspberry-Pi-Python-Code/Adafruit_CharLCDPlate
2. sudo python LCDtest.py
3. The backlight turns on and off, but NO characters are displayed

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brent_1
 
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Re: 16x2 Blue LCD Kit for raspi

Post by brent_1 »

The 4 connected pins on the MCP chip are A0, A1, A2, and #RESET.

Does it make sense that these would be tied together?

Who has a working board that they can disconnect from the rPi and test continuity on these pins?

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Re: 16x2 Blue LCD Kit for raspi

Post by brent_1 »

I got it working (for a while) by:

1. Unplugging the camera
2. Turning up the contrast all the way

It then worked for a few hours (using the Python library).

But overnight, it stopped working. And now the rPi won't boot while the LCDPlate it is attached.

Has anyone gotten this to work on a system that has the Camera enabled?

Any stability issues?

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Re: 16x2 Blue LCD Kit for raspi

Post by adafruit_support_bill »

The 4 connected pins on the MCP chip are A0, A1, A2, and #RESET.

Does it make sense that these would be tied together?
Yes. A0-A2 are the address pins and are all tied low by default. Based on the readings shown on the photo, the original issue was probably a simple contrast pot adjustment. Not sure about the possible camera interaction. Can you post a clearer shot of the soldering on the LCD display? Looks like there might be some issues there.

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Re: 16x2 Blue LCD Kit for raspi

Post by brent_1 »

I found the reason for the strange behavior:
The Up switch has < 1V on the high side - so it's always in the down state:
Image

The other switches pull 5V down to 0, but the Up switch starts out at only 2V, then decreases, so at first, it works. But then after approx. 1 minute of being attached to a running rPi, it is always "on", given that it's voltage is now under the threshold (actual voltage is < .5V). That translates to a a LOW on GPA3. Which is interpreted as button Up *always* in the depressed state.
Image
Image

The LCD Connections, as requested:
Image
Image

What could be pulling the Up button supply down? (The other buttons are fine -- HIGH until pressed to pull them LOW). And why would it get worse as the board warms up (over 60 seconds, you can see the voltage level drop on the Oscilloscope.)?

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Re: 16x2 Blue LCD Kit for raspi

Post by adafruit_support_bill »

I found the reason for the strange behavior:
The Up switch has < 1V on the high side - so it's always in the down state:
...
the Up switch starts out at only 2V, then decreases, so at first, it works. But then after approx. 1 minute of being attached to a running rPi, it is always "on", given that it's voltage is now under the threshold (actual voltage is < .5V).
I suppose it could be a bad switch, but the gradual decline sounds more like a bad pin on the MCP23017 port expander chip. If you contact [email protected] with a link to this thread, we can send you out a replacement.

Hint: to remove the old chip, first clip all the legs off, then desolder them one-by-one.

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