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.
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 |