For refreshing the whole frame.

It means, if you have 4 rows and change rows in each timer interrupt, the interrupt should occur 4 times faster than that.

But 50Hz is too little for LEDs. They don't have the persistence (decay) that the luminophor (phosphor) in TV screen has. The recommended absolute minimum is around 70Hz, but I'd recommend you to play with that frequency and try yourself (it's part of the fun). If you look at it from the "corner of your eye", you'll see more flicker than if you look straighn (this has to do with the "construction" of the eye - it's a wonderful piece of "engineering"... :-) ).

However, as many times said here, don't use LED matrix multiplexing if you can avoid it. It is OK for you to LEARN, but there are too many problems with this approach in real life. Use the long shiftregister approach rather (have a look at 74HC595), or any other ports expansion approach (see http://www.8052.com/faqs.phtml?FAQ=126158 for an overview).


JW