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:

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	Cap. (XTAL1/TOSC1)	Cap. (XTAL2/TSOC2)
System Osc.	18pF	8pF
Timer Osc.	6pF	6pF

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

C=2CL−Cs

where:

Ce - is optional external capacitors as described in Figure 1.
Ci - is the pin capacitance in the above table.
CL - is the load capacitance for a 32.768kHz crystal specified by the crystal vendor.
C_S - is the total stray capacitance for one TOSC pin.

Crystals specifying a load capacitance (CL) higher than 6pF require external capacitors applied as described in Low Power Crystal Oscillator.

The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to 0110 or 0111.

Table 3. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
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:

This option should only be used if frequency stability at start-up is not important for the application

Start-up times are determined by the SUT Fuses as shown in the following table.

Table 4. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
SUT[1:0]	Additional Delay from Reset (V_CC = 5.0V)	Power Conditions
00	14CK	BOD enabled
01	14CK + 4.1 ms	Fast rising power
10	14CK + 65 ms	Slowly rising power
11	Reserved