Hopefully someone can give me a tip. I'm completely lost on what's happening. I have the following problem. Using a 87C51RC processor I'm using 2 timers with interrupt, Serial comm with interrupt and the PCA timer with interrupt. The problem is that, while there is no code which generates a serial interrupt (nothing is written to SBUF) the TI flag is set after the Timer0 interrupt has been processed. In other words, after a Timer0 interrupt the TI flag is set for no apparent reason. I read after doing a search on the forum that sometimes a timing issue can cause a problem. after building several loops to reduce speed, and reducing the level of optimization. I still have no idea where to look. I would greatly appreciate any help on this issue. I am using: C51 Ver. 7.01 BL51 Ver. 5.01 uVision2 Ver 2.31 Kind regards John Garrelts