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Nokia 5110 LCD query.

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Nigel Goodwin

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I'm currently playing with a Nokia style graphic LCD, which is a 3.3V 84x48 monochrome display.

As I was initially using it with a 5V 16F1827, I mounted the display on a board with a 3.3V regulator, and fitted 10K series resistors in the connections, for 5V feeds as suggested in various places on the net. I've got this working perfectly, and am very happy with it.

Next I connected the board to a 3.3V 24F board, and I couldn't get it to work?.

Sticking the scope on the data and clock pins the SPI signals on the 5510 were very small (about 1V p-p), and triangular rather than square, but it was fine on the input side of the 10K resistors. So I simply shorted out the two 10K's on CLK and SDI, and it works perfectly now.

I'm just wondering if anyone has any idea why the 10K's caused so much trouble with 3.3V signals?
 

JimB

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My quick guess is that the 10k resistors in conjunction with the input capacitance of the LCD made a low pass filter.
Thus removing the high frequency components of the signals from the PIC, hence the triangle(ish) wave rather than a square wave.

JimB
 

spec

Well-Known Member
Most Helpful Member
I'm currently playing with a Nokia style graphic LCD, which is a 3.3V 84x48 monochrome display.

As I was initially using it with a 5V 16F1827, I mounted the display on a board with a 3.3V regulator, and fitted 10K series resistors in the connections, for 5V feeds as suggested in various places on the net. I've got this working perfectly, and am very happy with it.

Next I connected the board to a 3.3V 24F board, and I couldn't get it to work?.

Sticking the scope on the data and clock pins the SPI signals on the 5510 were very small (about 1V p-p), and triangular rather than square, but it was fine on the input side of the 10K resistors. So I simply shorted out the two 10K's on CLK and SDI, and it works perfectly now.

I'm just wondering if anyone has any idea why the 10K's caused so much trouble with 3.3V signals?
It is due to the input capacitance of the display pins Nigel.

The input capacitance of your scope probe makes it look worse though.

10K resistors are one heck of a high source impedance. As a result, the signal will not meet the rise and fall times of logic circuits, causing all sorts of nastiness.

Try 22p or so peaking capacitors in parallel with the 10K resistors. Adjust the capacitance until you get a nice square wave, just like you do with a scope probe.

But it is best to remove the 10K resistors from both circuits and replace them in the first circuit by low-impedance-drive level translators, as discussed @, http://www.electro-tech-online.com/threads/interfacing-to-displays-with-mcu-sbc.149299/#post-1277080

spec

(crossed posts Jim :))
 
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
My quick guess is that the 10k resistors in conjunction with the input capacitance of the LCD made a low pass filter.
Thus removing the high frequency components of the signals from the PIC, hence the triangle(ish) wave rather than a square wave.
Hi Jim, I'm aware of the capacitance and resistance potentially creating a lo-pass filter - my confusion is that it doesn't happen with the 5V feed from the 16F1827, only the 3.3V feed from the 24F.

However, thinking on, the 24F is running faster - both are using the SPI hardware, but the 24F SPI is running at 4MHz (the maximum for the 5110), while the 1827 is running at 1MHz (the maximum for the 1827 with the 4MHz clock I'm using).

I'll drop the SPI speed down on the 24F, and see how it works then.

Thanks for the clue, and Spec as well.
 

Nigel Goodwin

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Most Helpful Member
OK, removed my shorting links across the 10K's.

Tried it again, at 4MHz to confirm it still failed, it did.

So I dropped it to 500KHz, where it works fine - tried it at 2MHz, still worked fine - so alls good :D

Thanks again.

BTW, the reason for the 10K's was a simple way to make the board (which is compatible with my tutorial boards) work on either 16F or 24F boards, it has a jumper to select power either direct coming in (with a 3.3V board) or via a 3.3V regulator (with a 5V board). If anyone is interested?, here's the actual board layout - initially I had CE connected as well, but I needed 3 portB I/O lines available for a specific project I'm working on - I did consider removing the LED switching, but decided to leave for incase I needed a fourth I/O :D

Incidentally, in case people aren't aware, there are a number of different pinout configurations for these 5110 LCD modules - so you need to be careful which one you get (this one came from http://www.hobbytronics.co.uk - as I wanted one in a hurry, and I'm still waiting for my much cheaper Chinese ones :D).

5110.png
 

dr pepper

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Dont know how long it'd last but these 5110 displays work on 5v power and i2c lines, you just need to twiddle with the bias register.
 
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