74.7.8 32.768 kHz Crystal Oscillator
| Symbol | Parameter | Conditions | Min | Max | Unit | |
|---|---|---|---|---|---|---|
| VBAT | Supply voltage range (VBAT)(1) | – | VIT- | 3.6 | V | |
| IVBAT | Current consumption (VBAT)(2) | – | – | 3.1 | μA | |
| tSTART | Start-up time(2) | RS(3) < 50 kΩ | CM = 0.6fF | – | 3.7 | s |
| CM = 3fF | – | 0.8 | s | |||
| RS(3) < 90 kΩ | CM = 0.6fF | – | 5.0 | s | ||
| CM = 3fF | – | 1.0 | s | |||
| fOSC | Operating frequency | – | 32.768 | kHz | ||
| Duty | Duty cycle(2) | – | 40 | 60 | % | |
| RF | Internal resistor(2) | Between XIN32 and XOUT32 | 6 | MΩ | ||
| CPARA32K | Internal parasitic capacitance(2) | Between XIN32 and XOUT32 | 2 | pF | ||
Note:
- In this table, VIT- refers to the negative-going input threshold voltage of the VBAT POR (see Table 74-49). Operation of the device backup section is granted down to the VBAT POR VIT- threshold.
- Simulation data
- RS is the crystal’s equivalent series resistor.
CLEXT32K = 2x(CCRYSTAL – CPARA32K – CPCB / 2)
where CPCB is the single-ended (ground-referenced) parasitic capacitance of the printed circuit board (PCB) on XIN32 and XOUT32 tracks. As an example, if the crystal is specified for a 12.5 pF load, with CPCB = 1 pF (on XIN32 and on XOUT32), CLEXT32K = 2 x (12.5 - 2 - 0.5) = 20 pF.
The following table summarizes recommendations for 32.768 kHz crystal selection.
| Symbol | Parameter | Conditions | Min | Max | Unit |
|---|---|---|---|---|---|
| CCRYSTAL | Crystal load capacitance | As specified by the crystal manufacturer | 6 | 12.5 | pF |
| RS | Equivalent series resistor | – | – | 90 | kΩ |
| CM | Motional capacitance | – | 0.6 | 3 | fF |
| CSHUNT | Shunt capacitance | – | 0.6 | 2 | pF |
| PON | Drive level | – | 0.2 | – | μW |