Datasheet

Table 11-6. Maximum ESR Recommendation for 32.768 kHz Crystal
Crystal CL [pF] Max. ESR [kΩ]
(1)
6.5 75
9.0 65
12.5 30
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 11-7. Capacitance for Low Frequency Oscillator
32 kHz Osc. Type Internal Pad Capacitance
(XTAL1/TOSC1)
Internal Pad Capacitance
(XTAL2/TOSC2)
C
i
of system oscillator (XTAL pins) 18 pF 8 pF
C
i
of timer oscillator (TOSC pins) 18 pF 8 pF
The capacitance (C
e
+C
i
) needed at each TOSC pin can be calculated by using:


+ 

= 2

 

where:
• C
e
- is optional external capacitors. (= C
1
, C
2
as shown in Clock Source Connections.)
• C
i
- is the pin capacitance in Table 11-3.
• C
L
- is the load capacitance specified by the crystal vendor.
• C
S
- is the total stray capacitance for one XTAL pin.
Crystals specifying a load capacitance (CL) higher than 6 pF require external capacitors applied as
described in Clock Source Connections.
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 11-8. Start-Up Times for the Low Frequency Crystal Oscillator Clock Selection - SUT Fuses
SUT[1:0] Additional Delay from Reset (V
CC
= 5.0V) Recommended Usage
00 19CK Fast rising power or BOD enabled
01 19CK + 4.1 ms Slowly rising power
10 19CK + 65 ms Stable frequency at start-up
11 Reserved
ATmega328PB
System Clock and Clock Options
© 2018 Microchip Technology Inc.
Datasheet Complete
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