ATmega32 CKSEL Options

This fuse bit setting determines the clock source and clock type. This is where many people make mistakes and get the settings wrong. It is very important to understand the difference between a ceramic resonator and a crystal. Generally a crystal will be packaged in a metal can and have two terminals. A ceramic resonator will look like a ceramic capacitor but will have three terminals.

Another common mistake is choosing the 'External Clock' Option. The correct option for a 16 MHz external crystal for my development system is in green.

External Crystal or Ceramic Resonator

External Low-frequency Crystal

External RC Oscillator

Calibrated Internal RC Oscillator

External Clock

From the tables above we can see that for an external 16 MHz crystal, a setting of binary 1111 is needed which means no ticks in any of the boxes CKSEL3, CKSEL2, CKSEL1, and CKSEL0.

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