In a design with scanned matrix of LEDs, I have used ULN2803 as the cathode drivers. I find that the circuit works perfect with Atmel 89C52 and Siemens 80C535. But with 89C52BP of Philips the brightness of LEDs is poor and ocassionally the program hangs. With 89C52UBPN, an earlier Philips derivative the problem is a little less severe. What could be the reason? Also the brightness is not bad if I use a RD2(clk/12) instead of 89C52BP, but still not as good as that with the Atmel device. If I put a pullup network at the input of the ULN2803 the brightness improves with the Philips chips. The circuit has a BOR chip and these are ideal lab conditions.