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Electronics => Beginners => Topic started by: alank2 on August 30, 2014, 07:00:08 pm

Title: Do most LCD modules need a LPF on the contrast pin if fed by a uc PWM pin?
Post by: alank2 on August 30, 2014, 07:00:08 pm
Do most LCD modules need a LPF on the contrast pin if fed by a uc PWM pin?
Title: Re: Do most LCD modules need a LPF on the contrast pin if fed by a uc PWM pin?
Post by: bobcat on August 30, 2014, 07:07:52 pm
Yes. computed low pass filter at about 1/10 PWM frequency or lower
Title: Re: Do most LCD modules need a LPF on the contrast pin if fed by a uc PWM pin?
Post by: alank2 on August 30, 2014, 07:18:20 pm
Quote from: bobcat on August 30, 2014, 07:07:52 pm
Yes. computed low pass filter at about 1/10 PWM frequency or lower

Help me out, how would I do that?  The PWM is 64kHz.
Title: Re: Do most LCD modules need a LPF on the contrast pin if fed by a uc PWM pin?
Post by: alank2 on August 30, 2014, 08:02:58 pm
Hi,

So I picked a 250ohm resistor and 0.1u cap and it flattens it out pretty nicely.

Here is my question though - is it necessary?

I can run the PWM at 23.4% using the LPF for the display to look great, pixels change state quickly and it looks great.

If I skip the LPF and turn the PWM up about 10% to 33.2%, it looks the same.

Is the LPF really necessary?  I tried a couple of different displays and they both seem to work fine, can I save the two components or is there really some reason I should not?

Thanks,

Alan
Title: Re: Do most LCD modules need a LPF on the contrast pin if fed by a uc PWM pin?
Post by: rob77 on August 30, 2014, 08:59:50 pm
consult your LCD's datasheet to figure out whether or not it's necessary.
Title: Re: Do most LCD modules need a LPF on the contrast pin if fed by a uc PWM pin?
Post by: alank2 on August 31, 2014, 02:47:42 am
I've looked at a number of datasheets and most barely mention the contrast pin.

I tried a few LPF's, but I found one issue with them is that even if I feed a 0 PWM, it wouldn't bring it down to quite 0v and some lcd modules are touchy about the voltage being low.  It was around 500mV a few that I tried, I wonder if the resistor wasn't strong enough from the uC to bring it down to ground.

I then tried doing PWM directly to the LCD and at 1kHz, it was certainly not smooth, you could see it as flickering in the display.  At 8kHz though, you can't see it at all and you can go all the way down to 0v.
Title: Re: Do most LCD modules need a LPF on the contrast pin if fed by a uc PWM pin?
Post by: rob77 on August 31, 2014, 08:47:33 am
most of the LCDs have the contrast voltage specified as DC voltage, so i would definitely assume it's DC for all LCDs even if it's not mentioned in the datasheet. the cost of that RC low-pass (parts + PCB footprint) is half a bee's dick ;) so i see no reason to omit the low-pass filter.
Title: Re: Do most LCD modules need a LPF on the contrast pin if fed by a uc PWM pin?
Post by: bobcat on August 31, 2014, 11:52:24 am
The contrast pin is at the "bottom" of a multi step voltage divider. So if you measure from the contrast pin to ground, there will be a voltage. The reason for using DC versus PWM is that the biasing of LCD's is a complicated multi step waveform. It's not just "on" and "off". Without filtering the PWM, you are superimposing the PWM signal on to the bias waveform. This can decrease the life of the LCD or cause damage to the driver. 
Title: Re: Do most LCD modules need a LPF on the contrast pin if fed by a uc PWM pin?
Post by: alank2 on August 31, 2014, 01:15:55 pm
Quote from: bobcat on August 31, 2014, 11:52:24 am
It's not just "on" and "off". Without filtering the PWM, you are superimposing the PWM signal on to the bias waveform. This can decrease the life of the LCD or cause damage to the driver.

Well....that sounds like a good reason to implement the LPF then!  Thanks for the info!