I have used the 80C51FA CPU that has a PCA timer. The PCA supports a PWM mode that can be set up by the following:

MOV CMOD,#0 ; 1 Microsecond TIMEBASE @ 12 Mhz
MOV CCAPM0,#42H ; MODULE 0 IN PWM MODE
MOV CCAP0H,#128 ; 50% DUTY CYCLE
SETB CR ; START THE PCA TIMER

That's it. No additional timers, no interrupts, no CPU overhead.