sorry i messed up the test results, this is the result i got, tested it twice

test program
test:
high porta.0
pause 500
low porta.0
pause 500
goto test

result at osc 48
led on off duration is exactly 1 second

result at osc 20
led on off duration is twice

so i think define osc is correct.

i found this code(in document section) from sparkfun:http://www.sparkfun.com/products/9814
it is for adxl345 and uses spi but its in c and for atmega..