Table 5-3 XT1 Crystal Oscillator (Low Frequency)
over recommended ranges of supply voltage and operating free-air temperature (unless otherwise noted)(1)(2)
| PARAMETER |
TEST CONDITIONS |
MIN |
TYP |
MAX |
UNIT |
| fXT1, LF |
XT1 oscillator crystal, low frequency |
LFXTBYPASS = 0 |
|
32768 |
|
Hz |
| DCXT1, LF |
XT1 oscillator LF duty cycle |
Measured at MCLK,
fLFXT = 32768 Hz |
30% |
|
70% |
|
| fXT1,SW |
XT1 oscillator logic-level square-wave input frequency |
LFXTBYPASS = 1 (3)(4) |
|
32768 |
|
Hz |
| DCXT1, SW |
LFXT oscillator logic-level square-wave input duty cycle |
LFXTBYPASS = 1 |
40% |
|
60% |
|
| OALFXT |
Oscillation allowance for LF crystals (5) |
LFXTBYPASS = 0, LFXTDRIVE = {3},
fLFXT = 32768 Hz, CL,eff = 12.5 pF |
|
200 |
|
kΩ |
| CL,eff |
Integrated effective load capacitance(6) |
See (7) |
|
1 |
|
pF |
| tSTART,LFXT |
Start-up time (9) |
fOSC = 32768 Hz,
LFXTBYPASS = 0, LFXTDRIVE = {3},
TA = 25°C, CL,eff = 12.5 pF |
|
1000 |
|
ms |
| fFault,LFXT |
Oscillator fault frequency (10) |
XTS = 0(8) |
0 |
|
3500 |
Hz |
(1) To improve EMI on the LFXT oscillator, the following guidelines should be observed.
- Keep the trace between the device and the crystal as short as possible.
- Design a good ground plane around the oscillator pins.
- Prevent crosstalk from other clock or data lines into oscillator pins XIN and XOUT.
- Avoid running PCB traces underneath or adjacent to the XIN and XOUT pins.
- Use assembly materials and processes that avoid any parasitic load on the oscillator XIN and XOUT pins.
- If conformal coating is used, make sure that it does not induce capacitive or resistive leakage between the oscillator pins.
(3) When LFXTBYPASS is set, LFXT circuits are automatically powered down. Input signal is a digital square wave with parametrics defined in the Schmitt-trigger inputs section of this data sheet. Duty cycle requirements are defined by DCLFXT, SW.
(4) Maximum frequency of operation of the entire device cannot be exceeded.
(5) Oscillation allowance is based on a safety factor of 5 for recommended crystals. The oscillation allowance is a function of the LFXTDRIVE settings and the effective load. In general, comparable oscillator allowance can be achieved based on the following guidelines, but should be evaluated based on the actual crystal selected for the application:
- For LFXTDRIVE = {0}, CL,eff = 3.7 pF
- For LFXTDRIVE = {1}, 6 pF ≤ CL,eff ≤ 9 pF
- For LFXTDRIVE = {2}, 6 pF ≤ CL,eff ≤ 10 pF
- For LFXTDRIVE = {3}, 6 pF ≤ CL,eff ≤ 12 pF
(6) Includes parasitic bond and package capacitance (approximately 2 pF per pin).
(7) Requires external capacitors at both terminals to meet the effective load capacitance specified by crystal manufacturers. Recommended effective load capacitance values supported are 3.7 pF, 6 pF, 9 pF, and 12.5 pF. Maximum shunt capacitance of 1.6 pF. The PCB adds additional capacitance, so it must also be considered in the overall capacitance. Verify that the recommended effective load capacitance of the selected crystal is met.
(8) Measured with logic-level input frequency but also applies to operation with crystals.
(9) Includes start-up counter of 1024 clock cycles.
(10) Frequencies above the MAX specification do not set the fault flag. Frequencies between the MIN and MAX specification may set the flag. A static condition or stuck at fault condition sets the flag.