Just Curious, with the matrix LED sign you have all the LED anodes in a given ROW feeding to one transistor, and all the cathodes in a column feeding to another transistor.
Very close, I actually have the Anodes connected in rows then they connect to one of the 8-bit ports of the uC through resistors. (Since each port pin an handle 25mA, this enough to drive one LED)
As for the Cathodes, they are each connected in columns, then they each connect to the collector of a transistor.
I am guessing that the screen is thus refreshed by "scanning" through the entire panel form top left to bottom right sequentially ... and activating the Row and Column transistor for any given LED's if required.
each frame is drawn one column at a time. So the data will be placed on the data bus and then we turn on just one column of cathodes to display that data. Then we turn it off, send through the next byte of data and then activate the next successive column etc...
If so, what is the ON time for a single led?
guessing ( (1/refreshrate) / number of LEDs)
I am not actually sure about the on time, I just throw in a delay routine and see how it looks, if it is too 'flickery' then I need to decrease the delay, however, if they delay is not long enough the screen won't be as bright. so I may have to adjust the delay slightly 2 or 3 times to get it spot on.