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Topic: 8x8 RGB Matrix + 2x TLC5940 + 4051? (Read 5600 times) previous topic - next topic

lwhi

Thanks for the heads-up.  

Can anything be done to improve the speed of a PNP darlington array?  Could there be an application circuit, that would give it it a speed boost?

Just asking as I now have the UDN2981s .. but I guess I should look into the M54564P (that the rainbowduino uses).

I read the following note on the datasheet:

Quote
Turn-off delay is influenced by load conditions. Systems  applications well below the specified output loading may require timing considerations for some designs, i.e., multiplexed displays or  when used in combination with sink drivers in a totem pole configuration.

All unused inputs must be connected to ground. Pulldown resistors ([ch8776] 10 k[ch937]) are recommended for inputs that are allowed to float while power is being applied to VS.


Does this mean I should use a pull down to ground on the UDN2981 output pins?  Could this maybe help with timing?

lwhi

Quote
The UDN2981A will source enough current for this LED matrix, but the chip is quite SLOW. If you intend to do fast PWM, you'll see ghost images. The information from row N will 'bleed' to N-1, N-2 etc. You can see this effect in my avatar's icon.


What frequency were you using for updating? I'm trying to work out what would constitute fast PWM.

madworm

Oh my... I'd have to look at my code. You could do that as well ;-)

I was driving the 8x8 matrix with a screen refresh of about 50Hz to 60Hz with 15bit colors (5bit per color). Doing a back of the hand calculation gives 60x8(rows)x32(intensity steps) = 15.4kHz for the row switching. I don't have a scope at hand, so I can't measure it on the real thing ;-(

Anyhow, the turn-off delay for the UDN2981A is something about 5µs, and it was too long for that purpose. The source driver of the R-Duino has a turn-off delay of 1µs, and I haven't seen any ghosting on published images.
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lwhi

Hey thanks - I'll have a look over yr code - the back-of-hand calculation helps a lot.    

Do you think I should use extra resistors for the red component of the RGB matrix?




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