By way of the QJ.Net Forums, PSP coder jjinkou2 has released the Chess Clock homebrew. This PSP application simulates the the functions of a traditional chess clock. As for its controls:

start->start
L or R-> switch clock
Square->pause
Triangle -> stop

Jjinkou further notes that Chess Clock's background and figures customizable - they're just .png files. We imagine the serious chess fans among the PSP homebrew community will take a fancy to this application - it beats having to lug around a separate analog clock. There's note really much else to explain at this point, as the author designed this app to be simple and easy to to use. Make sure to visit jjinko's thread over at the QJ.Net Forums for extra pointers.