11.3 Low-Power Crystal Oscillator

This Crystal Oscillator is a low-power oscillator, with reduced voltage swing on the XTAL2 output. It gives the lowest power consumption, but is not capable of driving other clock inputs, and may be more susceptible to noise in noisy environments.

The crystal should be connected as described in Clock Source Connections. When selecting crystals, load capacitance must be taken into consideration. The capacitance (Ce+Ci) needed at each TOSC pin can be calculated by using:

Ce+Ci=2CLCs
where:
  • Ce - is optional external capacitors. (= C1, C2 as shown in the schematics.)
  • Ci - is the pin capacitance in the following table.
  • CL - is the load capacitance specified by the crystal vendor.
  • CS - is the total stray capacitance for one XTAL pin.
Table 11-3. Internal Capacitance of Low-Power Oscillator
32 kHz Osc. TypeInternal Pad Capacitance (XTAL1)Internal Pad Capacitance (XTAL2)
Ci of system oscillator (XTAL pins)18 pF8 pF

The Low-power Oscillator can operate in three different modes, each optimized for a specific frequency range. The operating mode is selected by the fuses CKSEL[3:1], as shown in the following table:

Table 11-4. Low-Power Crystal Oscillator Operating Modes(1)
Frequency Range
 [MHz]CKSEL[3:1](2)Absolute Limits for Total Capacitance of C1 and C2 [pF](4)
0.4 - 0.9100(3)
0.9 - 3.010112 - 22
3.0 - 8.011012 - 22
8.0 - 16.011112 - 22
Note:
  1. These are the recommended CKSEL settings for the different frequency ranges.
  2. This option should not be used with crystals, only with ceramic resonators.
  3. If the crystal frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be programmed in order to divide the internal frequency by 8. It must be ensured that the resulting divided clock meets the frequency specification of the device.
  4. When selecting the external capacitor value, the stray capacitance from the PCB and device should be deducted. The total load (Ce+Ci+Cs) on XTAL pins must not exceed 22 pF.

The CKSEL0 Fuse together with the SUT[1:0] Fuses select the start-up times, as shown in the following table:

Table 11-5. Start-Up Times for the Low-Power Crystal Oscillator Clock Selection
Oscillator Source/Power ConditionsStart-Up Time from Power-Down and Power-SaveAdditional Delay from Reset 
(VCC = 5.0V)CKSEL0SUT[1:0]
Ceramic resonator, fast rising power258 CK19CK + 4 ms(1)000
Ceramic resonator, slowly rising power258 CK19CK + 65 ms(1)001
Ceramic resonator, BOD enabled1K CK19CK(2)010
Ceramic resonator, fast rising power1K CK19CK + 4 ms(2)011
Ceramic resonator, slowly rising power1K CK19CK + 65 ms(2)100
Crystal Oscillator, BOD enabled16K CK19CK101
Crystal Oscillator, fast rising power16K CK19CK + 4 ms110
Crystal Oscillator, slowly rising power16K CK19CK + 65 ms111
Note:
  1. These options should only be used when not operating close to the maximum frequency of the device, and only if frequency stability at start-up is not important for the application. These options are not suitable for crystals.
  2. These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They can also be used with crystals when not operating close to the maximum frequency of the device and if frequency stability at start-up is not important for the application.