14.4 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 14-6. Maximum ESR Recommendation for 32.768kHz Crystal
Crystal CL [pF]Max. ESR [kΩ](1)
6.575
9.065
12.530
Note:
  1. Maximum ESR is typical value based on characterization.

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

Table 14-7. Capacitance for Low-Frequency Oscillator
32kHz Osc. TypeInternal Pad Capacitance (XTAL1/TOSC1)Internal Pad Capacitance (XTAL2/TOSC2)
System Osc.18pF8pF
Timer Osc.18pF8pF

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

Ce+Ci=2CL−Cs
where:
  • Ce - is optional external capacitors. (= C1, C2 as shown in Figure 14-2)
  • Ci - is the pin capacitance in Table 14-3.
  • CL - is the load capacitance specified by the crystal vendor.
  • CS - is the total stray capacitance for one XTAL pin.

Crystals specifying a load capacitance (CL) higher than 6pF require external capacitors applied as described in Figure 14-2.

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 14-8. 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
0019CKFast rising power or BOD enabled
0119CK + 4.1msSlowly rising power
1019CK + 65msStable frequency at start-up
11Reserved
Table 14-9. Start-Up Times for the Low-frequency Crystal Oscillator Clock Selection - CKSEL Fuses
CKSEL[3:0]Start-Up Time from 
Power-down and Power-SaveRecommended Usage
0100(1)1K CK
010132K CKStable frequency at start-up
Note:
  1. This option should only be used if frequency stability at start-up is not important for the application.