Low-frequency Crystal Oscillator

To use a 32.768kHz watch crystal as the clock source for the device, the Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “1001”. The crystal should be connected as shown in Figure 1. By programming the CKOPT Fuse, the user can enable internal capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors. The internal capacitors have a nominal value of 36pF.

When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in the table below.

Table 1. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1:0 Start-up Time from 
Power-down and 
Power-save Additional Delay 
from Reset 
(VCC = 5.0V) Recommended Usage
00 1K CK(1) 4.1ms Fast rising power or BOD enabled
01 1K CK(1) 65ms Slowly rising power
10 32K CK 65ms Stable frequency at start-up
11 Reserved
Note: 1. These options should only be used if frequency stability at start-up is not important for the application.