thank you so much.i understand well…. sir i tried with 8051,its ok..but in ARM7 LPC2148 controller, there are two frequencies.one is crystal frequency =12MHz and PCLK(pheripheral clock)=30MHz, where (VPBDIV=0X02 i.e 1/4 of the default PLL output=60MHz )….now my doubt is which frequency we have to consider in performing PWM in this controller and let me know how? how frequency and Ton and Toff are related and with which frequency?