Help with timer LPC2106

Hello,

I am using an LPC2106 CPU running at a speed of 10MHz, I would use a timer that counts in periods of 1 microseconds just as the system clock without interruption. I can check the system's a good time when I use a period of 1 millisecond but with 1 microseconds the error is very big, around 40%. Can anyone help me with that?

Best Regards,

Bruno
CIn/UFPE Brazil