Trigger Question

I'm trying to write a trigger that updates a date_changed field on a
record anytime that record is updated. I have a function written, and
the trigger created, but everytime I update the record, I get a
recursion limit error. It appears that the action performed by my
trigger is causing the trigger to fire. How do I avoid that. For the
record, here's my function (modeled after a trigger that works on SQL
Server. I'm pretty much a noob with plpgsql :)

declare begin
update unit_specification set date_changed = now() from
unit_specification us where us.id = NEW.id;
RETURN NEW;
end;

With the trigger created with

CREATE TRIGGER update_changed_ts AFTER INSERT OR UPDATE
ON unit_specification FOR EACH ROW
EXECUTE PROCEDURE public.update_unit_spec_changed_date();

Thanks for the help. :)

-----
Jason Lee, SCJP
Senior Software Engineer
http://www.iec-okc.com <http://www.iec-okc.com/>