Low Frequency Crystal Oscillator

The Low-frequency Crystal Oscillator is optimized for use with a 32.768kHz watch crystal. When selecting crystals, load capacitance and crystal’s Equivalent Series Resistance (ESR) must be taken into consideration. Both values are specified by the crystal vendor. The oscillator is optimized for very low power consumption, and thus when selecting crystals, consider the Maximum ESR Recommendations:

Table 1. Maximum ESR Recommendation for 32.768kHz Crystal
Crystal CL [pF] Max. ESR [kΩ](1)
6.5 75
9.0 65
12.5 30
Note:
  1. 1. Maximum ESR is typical value based on characterization.

The Low-frequency Crystal Oscillator provides an internal load capacitance at each TOSC pin:

Table 2. Capacitance for Low-Frequency Oscillator
32kHz Osc. Type Internal Pad Capacitance (XTAL1/TOSC1) Internal Pad Capacitance (XTAL2/TOSC2)
System Osc. 18pF 8pF
Timer Osc. 18pF 8pF

The capacitance (Ce+Ci) needed at each TOSC pin can be calculated by using:

Ce+Ci=2CLCs
where:

Crystals specifying a load capacitance (CL) higher than 6pF require external capacitors applied as described in Figure 1.

The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to '0110' or '0111',and Start-Up times are determined by the SUT Fuses, as shown in the following two tables.

Table 3. Start-Up Times for the Low-frequency Crystal Oscillator Clock Selection - SUT Fuses
SUT[1:0] Additional Delay from Reset (VCC = 5.0V) Recommended Usage
00 19CK Fast rising power or BOD enabled
01 19CK + 4.1ms Slowly rising power
10 19CK + 65ms Stable frequency at start-up
11 Reserved
Table 4. Start-Up Times for the Low-frequency Crystal Oscillator Clock Selection - CKSEL Fuses
CKSEL[3:0] Start-Up Time from 
Power-down and Power-Save Recommended Usage
0100(1) 1K CK  
0101 32K CK Stable frequency at start-up
Note:
  1. 1.This option should only be used if frequency stability at start-up is not important for the application.