Hi everyone,

I have a strrrrange problem with a ledmatrix I built! I made a 16x16 display with green leds, which is capable of showing 4 colors (intensities) at once.System is driven by a 8052 and works NICE, as long as the clock is kept slow...

With speeds of 20Khz The panel barely flickers, and the picture is nice where i want it, but when I increase the clock to 400 Khz, the data seems to 'add up' from the previous dot. What I mean is that when i show a black screen, with 1 bright dot in the middle, it shows the bright dot correct, but the dot being shown next seems to mirror the voltage of the previous bright dot. And it becomes worse when I increase the speed more : at 1 Mhz It mirrors in the 4 next dots!!

I suspect the problem to be somewhere like capacitive inductance, but I am not sure.
I made the display driver out of a counter for the dot-address, which is demultiplexed in rows and columns. the outputs of the demultiplexers are then connected with the base of transistors type bc857 (smd). The colordata is retrieved from a ram-ic, fed into a demultiplexer too,also connected to transistors, of wich the collector is limited by resistors, defining the color. The current running through a led at full color is 60ma, much, but short enough not to burn it and still bright. Could it be that a capacity exists over the resistors and that the old 'voltage' is fed trough the next leds at high speeds?

phew, sorry for the long text, I couldn't explain it shorter...

Thanx