Lpc2000

Hi! I am building a board featuring the LPC2138 µC. This board will 
basically need to interface with three serial devices (two via 
internal UARTs and a third via MAX3100, though suggestions are 
welcomed about this). This device must be very low power; it should 
wake up only when needed, which is: 1) there's data available from 
any of the three ports, 2) there's data waiting to be transmitted on 
any of the ports and the port becomes free and 3) a periodical 
overall check (every second or so). This means that theoretically the 
processor can enter the idle state quite often. I'm looking for some 
(free if possible) multithreaded OS that would handle the transition 
to the processor's idle state on traditionally blocking calls like 
fread() and fwrite(), expecting to wake up on interrupt; also, some 
preemptive multitasking should be supported, in case some thread 
enters an exceptionally long calculating task. Probably there's no 
known OS that directly supports this; in this case, I'd like to know 
which other OS might be a good starting point to accomplish this by 
making the modifications myself. I'm aiming to use the GNU libraries 
(I ignore how difficult or messy would it be to modify them to 
support this). Finally, it is my understanding that the VERY low 
power "power down" mode would not be useful for my application, since 
the peripherals (therefore the UARTs and SPI) are shut down in this 
mode. Any advise or hint on these subjects will be greately 
appreciated.

Guille