For timing insensitive applications, the external RC configuration shown in Figure 12 can be used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at least 22 pF. By programming the CKOPT Fuse, the user can enable an internal 36 pF capacitor between XTAL1 and GND, thereby removing the need for an external capacitor. For more information on Oscillator operation and details on how to choose R and C, refer to the External RC Oscillator application note.
Добавлено спустя 3 минуты 51 секунду: В виду прокопипастеного у самого появился вопрос, ктонибудь побовал менять частоту АВРа на ходу - как работало?