External RC Oscillator

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.

Table 1. External RC Oscillator Operating Modes
CKSEL<3:0> 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.

Table 2. Start-up Times for the External RC Oscillator Clock Selection
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.