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]
9.0	65
12.5	30

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

Table 2. Internal capacitance of Low-Frequency Oscillator
32kHz Osc. Type	Internal Pad Capacitance (XTAL1/TOSC1)	Internal Pad Capacitance (XTAL2/TOSC2)
C_i of system oscillator (XTAL pins)	6pF	6pF
C_i of timer oscillator (TOSC pins)	4pF	4pF

The capacitance (C_e+C_i) needed at each XTAL/TOSC pin can be calculated by using:

Ce+Ci=2CL−Cs

where:

C_e - is optional external capacitors. (=C₁, C₂ as shown in Figure 1)
C_i - is the pin capacitance in Table 2.
C_L - 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 (C_L) higher than 6pF require external capacitors applied according to above formula.

When this oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in the following table.

Table 3. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Power Conditions	Start-up Time from Power-down and Power-save	Additional Delay from Reset (V_CC = 5.0V)	CKSEL[0]	SUT[1:0]
BOD enabled	1K CK	14CK⁽¹⁾	0	00
Fast rising power	1K CK	14CK + 4ms⁽¹⁾	0	01
Slowly rising power	1K CK	14CK + 65ms⁽¹⁾	0	10
Reserved			0	11
BOD enabled	32K CK	14CK	1	00
Fast rising power	32K CK	14CK + 4ms	1	01
Slowly rising power	32K CK	14CK + 65ms	1	10
Reserved			1	11

Note:

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