58.7.7 32.768 kHz Crystal Oscillator
| Symbol | Parameter | Conditions | Min | Typ | Max | Unit | |
|---|---|---|---|---|---|---|---|
| VDDBU | Supply voltage range (VDDBU) | – | VIT-(1) | – | 3.6 | V | |
| IVDDBU | Current consumption (VDDBU)(2) | – | – | – | 3.1 | μA | |
| tSTART | Startup time(2) | ESR < 50 kΩ | CM = 0.6 fF | – | – | 3.7 | s |
| CM = 3 fF | – | – | 0.8 | s | |||
| ESR < 90 kΩ | CM = 0.6 fF | – | – | 5.0 | s | ||
| CM = 3 fF | – | – | 1.0 | s | |||
| fOSC | Operating frequency | – | 32.7 | – | 32.8 | kHz | |
| Duty | Duty cycle | – | 40 | 50 | 60 | % | |
| CPARA32K | Internal parasitic capacitance(2) | Between XIN32 and XOUT32 | – | 1.7 | – | pF | |
Note:
- In this table, VIT- corresponds to the negative-going input threshold voltage of VDDBU POR (see the table VDDBU POR Characteristics). Operation of the device backup section is granted down to the VDDBU POR VIT- threshold.
- Simulation data
The 32.768 crystal oscillator supports a Bypass mode. See Input AC Characteristics.
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 - 1.7 - 0.5) = 20.6 pF.
The table below summarizes recommendations for 32.768 kHz crystal selection.
| Symbol | Parameter | Conditions | Min | Typ | Max | Unit |
|---|---|---|---|---|---|---|
| CCRYSTAL | Crystal load capacitance | As specified by the crystal manufacturer | 6 | – | 12.5 | pF |
| RS | Equivalent series resistor | – | – | 50 | 90 | kΩ |
| CM | Motional capacitance | – | 0.6 | – | 3 | fF |
| CSHUNT | Shunt capacitance | – | 0.6 | – | 2 | pF |
| PON | Drive level | – | 0.2 | – | – | μW |