For timing insensitive applications, the external RC configuration shown in the figure below can be used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at least 22pF. By programming the CKOPT Fuse, the user can enable an internal 36pF capacitor between XTAL1 and GND, thereby removing the need for an external capacitor.
Figure 1. External RC Configuration
The Oscillator can operate in four different modes, each optimized for a specific frequency range. The operating mode is selected by the fuses CKSEL3:0 as shown in the following table.
| CKSEL3:0 | Frequency Range (MHz) |
|---|---|
| 0101 | 0.1 - 0.9 |
| 0110 | 0.9 - 3.0 |
| 0111 | 3.0 - 8.0 |
| 1000 | 8.0 - 12.0 |
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in the table below.
| SUT1:0 | Start-up Time from Power-down and Power-save | Additional Delay from Reset (VCC = 5.0V) | Recommended Usage |
|---|---|---|---|
| 00 | 18 CK | – | BOD enabled |
| 01 | 18 CK | 4.1ms | Fast rising power |
| 10 | 18 CK | 65ms | Slowly rising power |
| 11 | 6 CK(1) | 4.1ms | Fast rising power or BOD enabled |
Note: 1. This option should not be used when operating close to the maximum frequency of the
device.