Lpc2000

A couple of suggestions/options:
1) Run in ARM mode - faster w/ 32-bit memories
2) Possibly run a multiplier of 2X => 28+MHz CPU speed will
   help and still not be as current consuming as full speed.
3) Check the manner in which UART register addresses are used in
   the ISR.  A simple #define of each register address will
   typically cause a 32-bit cpu register load, then a 0 offset
   access of the UART register.  If several UART registers are
   accessed, typedef a struct for the UART register group then
   declare and use a pointer to the UART register group in the ISR.
   The compiler will then use offset addressing to access specific
   UART registers. For examples, look at the LPC210x.h & associated
   files I uploaded a couple of weeks ago.  The resulting executible
   will be potentially smaller and faster.

Regards
-Bill Knight
R O SoftWare