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Electronics => Projects, Designs, and Technical Stuff => Topic started by: davorin on April 27, 2018, 01:59:04 pm

Title: RGB LED matrix chain limits
Post by: davorin on April 27, 2018, 01:59:04 pm

EHLO (o;

I guess some or many have used or use the RGB matrix library from https://github.com/hzeller/rpi-rgb-led-matrix (https://github.com/hzeller/rpi-rgb-led-matrix)

The author states that it would be theoretically possible to drive a chain of 32 * 32x32 panels at 100Hz refresh rate and 24bit colors...

I have some 64x64 panels here I want to use...but when calculating the required clock rate I get:

Code: [Select]

Refresh: 60Hz
Rows: 32 (two rows are lit at the same time)
Columns: 64
Clock rate for simple on/off: 122.88kHz

Now when adding colours levels with 8 bit PWM the clock rate needs to be multiplied by 256, leading to a clock at 31.46MHz, which is already above the DP5220X shifter specs...

So how would it be possible at all to get a larger chain setup while keeping a good refresh rate?

Or did I calculate something completely wrong here? (o;

Title: Re: RGB LED matrix chain limits
Post by: german77 on April 27, 2018, 04:41:08 pm

I never heard of or tested this library.

But talking theoretically if you can only light a led at a time you need a frequency of 245.7kHz in order to only turn on an off a led. With pwm this value will get to 62.9MHz which is impractical.

This library is for raspberry which from were I know it has 2 pwm channels. That's why you get 31MHz. You should be able to lower the PWM resolution to lower the clock speeds. With 6 bit pwm you would have 64 different values, a clock rate of 7.86MHz and I bet that would look just as fine as the 8 bit pwm.

Title: Re: RGB LED matrix chain limits
Post by: davorin on April 28, 2018, 10:37:43 am

Actually PWM is only used for switching GPIO pins on/off...not for brightness control as it won't work..as you would control the brightness of two rows at once ( in case of the 64x64 panel).