10.3 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:
| 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:
| 32kHz Osc. Type | Internal Pad Capacitance (XTAL1/TOSC1) | Internal Pad Capacitance (XTAL2/TOSC2) |
|---|---|---|
| Ci of system oscillator (XTAL pins) | 6pF | 6pF |
| Ci of timer oscillator (TOSC pins) | 4pF | 4pF |
The capacitance (Ce+Ci) needed at each XTAL/TOSC pin can be calculated by using:
- Ce - is optional external capacitors. (=C1, C2 as shown in Figure 10-2)
- Ci - is the pin capacitance in Table 10-4.
- CL - is the load capacitance for a 32.768kHz crystal specified by the crystal vendor.
- CS - is the total stray capacitance for one TOSC pin.
Crystals specifying a load capacitance (CL) 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.
| Power Conditions | Start-up Time from Power-down and Power-save | Additional Delay from Reset (VCC = 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 | ||
- This option should only be used if frequency stability at start-up is not important for the application.