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](1) |
---|---|
6.5 | 75 |
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) |
---|---|---|
System Osc. | 18pF | 8pF |
Timer Osc. | 18pF | 8pF |
The capacitance (Ce+Ci) needed at each TOSC pin can be calculated by using:
Crystals specifying a load capacitance (CL) higher than 6pF require external capacitors applied as described in Figure 1.
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.
SUT[1:0] | Additional Delay from Reset (VCC = 5.0V) | Recommended Usage |
---|---|---|
00 | 19CK | Fast rising power or BOD enabled |
01 | 19CK + 4.1ms | Slowly rising power |
10 | 19CK + 65ms | Stable frequency at start-up |
11 | Reserved |
CKSEL[3:0] | Start-Up Time from Power-down and Power-Save | Recommended Usage |
---|---|---|
0100(1) | 1K CK | |
0101 | 32K CK | Stable frequency at start-up |