Data Sheet

1013
Atmel | SMART SAM D21 [DATASHEET]
Atmel-42181G–SAM-D21_Datasheet–09/2015
38.6.3 External Real Time 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 SAM D21R21 oscillator (not available in SAMR21E) is optimized for very low power consumption, hence close
attention should be made when selecting crystals, see Table 38-6 for maximum ESR recommendations on 9pF and
12.5pF crystals
.
The Low-frequency Crystal Oscillator provides an internal load capacitance of typical values available in Table 36-47,
“32kHz Crystal Oscillator Characteristics, Device Variant A,” on page 972. This internal load capacitance and PCB
capacitance can allow to use a Crystal inferior to 12.5pF load capacitance without external capacitors as shown in Figure
38-7.
Note: Maximum ESR is typical value based on characterization. These values are not covered by test limits in
production.
Figure 38-7. External Real Time Oscillator without Load Capacitor
However, to improve Crystal accuracy and Safety Factor, it can be recommended by crystal datasheet to add external
capacitors as shown in Figure 38-8.
To find suitable load capacitance for a 32.768kHz crystal, consult the crystal datasheet.
Figure 38-8. External Real Time Oscillator with Load Capacitor
Notes: 1. These values are given only as typical examples.
Table 38-6. Maximum ESR Recommendation for 32.768kHz Crystal
Crystal C
L
(pF) Max ESR [kΩ]
12.5 313
Table 38-7. External Real Time Oscillator Checklist
Signal Name Recommended Pin Connection Description
XIN32 Load capacitor 22pF
(1)(2)
Timer oscillator input
XOUT32 Load capacitor 22pF
(1)(2)
Timer oscillator output
XOUT32
XIN32
32.768kHz
XOUT32
XIN32
32.768kHz
22pF
22pF