Thanks for the replies.

I am currently refreshing the LEDs (128 of them), by writing out a serial stream of bits to 4094 shift-registers w/latches, then I strobe to latch the values. I can't really think of a way to vary the duty cycle using this arrangement. In my case I don't think flickering will be a problem, so I will try to reduce the refresh and see what happens.

This is my first 8051 project so I am taking it slow :)

Thanks again
John