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Topic: Graphical LCD (ST7920 Driver) - a warning and a question (Read 2372 times) previous topic - next topic

cptdondo

I just got a shipment of the generic "12864 128x64 Graphic Matrix LCD Module ST9720" screens from China.

The PSB pin is hardwired to Vdd, which means that if you attempt to put it into serial mode by taking PSB to ground, you short your powersupply to ground.  Gah.

Now for the question:

Is it possible to adjust contrast without a pot?  Can I run a PWM signal into V0?

floresta

#1
Aug 03, 2013, 06:15 pm Last Edit: Aug 03, 2013, 06:28 pm by floresta Reason: 1
Quote
The PSB pin is hardwired to Vdd, which means that if you attempt to put it into serial mode by taking PSB to ground, you short your powersupply to ground.  Gah.

Are you sure it is hardwired (connected directly) to Vdd and not 'pulled-up' via a resistor?

Quote
Is it possible to adjust contrast without a pot?  Can I run a PWM signal into V0?

That may be a problem since most GLCDs require a negative voltage for the contrast pin.

I just found this --> http://forum.arduino.cc/index.php?topic=22588.0 which may or may not be of some help.

Here's another one -->  http://www.bajdi.com/cheap-128x64-graphic-lcd-12864zw/ try a Google search for glcd psb pin.


Don

cptdondo


Quote
The PSB pin is hardwired to Vdd, which means that if you attempt to put it into serial mode by taking PSB to ground, you short your powersupply to ground.  Gah.

Are you sure it is hardwired (connected directly) to Vdd and not 'pulled-up' via a resistor?



Well, if I ground PSB and I power the screen up from a USB port, the port shuts down from current overload.  My fluke DVM says there's 0.2 ohms between Vcc and PSB.

And the screens I got last time work just fine.  So it's a defective run...  Fortunately I did not fry anything, just a few $ and some hours tracking it down.


Quote
Is it possible to adjust contrast without a pot?  Can I run a PWM signal into V0?

That may be a problem since most GLCDs require a negative voltage for the contrast pin.

I just found this --> http://forum.arduino.cc/index.php?topic=22588.0 which may or may not be of some help.

Here's another one -->  http://www.bajdi.com/cheap-128x64-graphic-lcd-12864zw/ try a Google search for glcd psb pin.


Don



Thanks, I'll look at those.  I'm not familiar with these LCDs.

TheCoolest

cptdondo:

0.2ohm would suggest that PSB is indeed shorted to VCC. I would suggest simply finding the trace and cutting it with a xaco knife, be careful not to cut the other traces. But then you will be able to either ground or apply vcc to the pin when needed.

cptdondo

Great minds think alike. Turns out that there was a jumper disguised as a resistor, shorting Vdd to PSB.  I removed it and now all the screens work.  :)

Just a "beware" on these cheap Chinese units; sometimes they do strange things.  All in all I'm quite happy with the quality and response of the direct-ship vendors but once in a while they do make life interesting.

floresta

Quote
Great minds think alike. Turns out that there was a jumper disguised as a resistor, shorting Vdd to PSB.

I suspect that the original intention was for a pull-up resistor to be installed but they put in a 0-ohm resistor instead.

Don

dc42

I've seen several of these units, all using the same PCB but populated differently. I have one with a tiny SMD contrast pot soldered on the back (the silk screen label is VR1) and positions R6 and R7 are open. Others have fixed resistors, and some have the contrast pin of the chip brought out to the connector.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

cider101

i have the same problem with such a display...do you remeber, which resistor caused to problem ?

dc42


i have the same problem with such a display...do you remeber, which resistor caused to problem ?


Please describe what problem you have, and post a photo of the back of the display.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

cider101

#9
Feb 02, 2014, 08:25 pm Last Edit: Feb 02, 2014, 08:35 pm by cider101 Reason: 1

Please describe what problem you have, and post a photo of the back of the display.


if i connect the PSB pin to GND, the backlight led goes of and the display gets warm/hot. I measured 5v from PSB to GND.

Here is a link to the backside of the PCB. On the front is nothing except the display, therefore i didnt take a picture
http://i1224.photobucket.com/albums/ee372/cider101/Misc/47c1839a-e114-4b78-b2c8-8eef0633b8ee.jpg~original

I tried to trace the PSB lead. I seems, it ends on the spot marked with the black arrow...
Thx for your help in advance!

dc42

Looking at the photo, it appears possible that PSB is connected to one side of R9, and R9 may be a 0 ohm resistor (with the other end presumably connected to +5V). Can you check this by visual inspection and with a multimeter? The ST7920 board I have is unfortunately different in that area and so not directly comparable.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

cider101

thx for your answer

i tried to mark some leads/traces near the PSB. But some connections/traces are just guesses since the white labeling covers them.
http://i1224.photobucket.com/albums/ee372/cider101/Misc/photo1.jpg~original

and here is the original pic.
http://i1224.photobucket.com/albums/ee372/cider101/Misc/photo.jpg~original

I'll try to take a high res picture with my camera tonight....


dc42

You may need to use a multimeter to check whether PSB is connected to R9. My guess is that PSB is connected to both R9 and R10, that the other side of R9 goes to +5V, and the other side of R10 goes to ground. So the display can be fixed in serial or parallel mode depending on which one is fitted, or can be selected via the PSB pin if neither is fitted. So if you want to use it in serial mode, you will have to unsolder R9.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

cider101

thanks a lot for your help!
I'll check it this evening  and post the results...

cider101


You may need to use a multimeter to check whether PSB is connected to R9. My guess is that PSB is connected to both R9 and R10, that the other side of R9 goes to +5V, and the other side of R10 goes to ground. So the display can be fixed in serial or parallel mode depending on which one is fitted, or can be selected via the PSB pin if neither is fitted. So if you want to use it in serial mode, you will have to unsolder R9.


thanks mate...i owe you a big pint :) I desolderer R9 and it's working now. Thanks again !

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