Datenblatt Mega16 Seite 23:

Table 2.
Device Clocking Options Select(1) Device Clocking Option CKSEL3..0
External Crystal/Ceramic Resonator 1111 - 1010
External Low-frequency Crystal 1001
External RC Oscillator 1000 - 0101
Calibrated Internal RC Oscillator 0100 - 0001
External Clock 0000

Probier doch mal 1001