Again, when debugging the RTC dialog shows this warning when setting the RTC CCR register to 0x11 (valid for the LPC2103 , but invalid for the LPC2106. You have entered a number that is out of range! The previous value will be restored. Maximum value = 0x0F For some reason (?) keil treats lpc2103 as an lpc2106 device (anyway it does in my evaluation package ). I discovered similar issues with the PINSEL registers. (Keil seems to ignore bugs reported by EVALUATION version users, never got any reply on my first report ).