Table 5-4 Low-Frequency Crystal Oscillator, LFXT(4)
over recommended ranges of supply voltage and operating free-air temperature (unless otherwise noted)
| PARAMETER |
TEST CONDITIONS |
VCC |
MIN |
TYP |
MAX |
UNIT |
| IVCC.LFXT |
Current consumption |
fOSC = 32768 Hz,
LFXTBYPASS = 0,LFXTDRIVE = {0},
TA = 25°C, CL,eff = 3.7 pF, ESR ≈ 44 kΩ |
3.0 V |
|
180 |
|
nA |
fOSC = 32768 Hz,
LFXTBYPASS = 0, LFXTDRIVE = {1},
TA = 25°C, CL,eff = 6 pF, ESR ≈ 40 kΩ |
|
185 |
|
fOSC = 32768 Hz,
LFXTBYPASS = 0, LFXTDRIVE = {2},
TA = 25°C, CL,eff = 9 pF, ESR ≈ 40 kΩ |
|
225 |
|
fOSC = 32768 Hz,
LFXTBYPASS = 0, LFXTDRIVE = {3},
TA = 25°C, CL,eff = 12.5 pF, ESR ≈ 40 kΩ |
|
330 |
|
| fLFXT |
LFXT oscillator crystal frequency |
LFXTBYPASS = 0 |
|
|
32768 |
|
Hz |
| DCLFXT |
LFXT oscillator duty cycle |
Measured at ACLK,
fLFXT = 32768 Hz |
|
30% |
|
70% |
|
| fLFXT,SW |
LFXT oscillator logic-level square-wave input frequency |
LFXTBYPASS = 1(5)(8) |
|
10.5 |
32.768 |
50 |
kHz |
| DCLFXT, SW |
LFXT oscillator logic-level square-wave input duty cycle |
LFXTBYPASS = 1 |
|
30% |
|
70% |
|
| OALFXT |
Oscillation allowance for LF crystals(9) |
LFXTBYPASS = 0, LFXTDRIVE = {1},
fLFXT = 32768 Hz, CL,eff = 6 pF |
|
|
210 |
|
kΩ |
LFXTBYPASS = 0, LFXTDRIVE = {3},
fLFXT = 32768 Hz, CL,eff = 12.5 pF |
|
|
300 |
|
| CLFXIN |
Integrated load capacitance at LFXIN terminal(6)(7) |
|
|
|
2 |
|
pF |
| CLFXOUT |
Integrated load capacitance at LFXOUT terminal(6)(7) |
|
|
|
2 |
|
pF |
| tSTART,LFXT |
Start-up time(2) |
fOSC = 32768 Hz,
LFXTBYPASS = 0, LFXTDRIVE = {0},
TA = 25°C, CL,eff = 3.7 pF |
3.0 V |
|
800 |
|
ms |
fOSC = 32768 Hz,
LFXTBYPASS = 0, LFXTDRIVE = {3},
TA = 25°C, CL,eff = 12.5 pF |
3.0 V |
|
1000 |
|
| fFault,LFXT |
Oscillator fault frequency(3)(1) |
|
|
0 |
|
3500 |
Hz |
(1) Measured with logic-level input frequency but also applies to operation with crystals.
(2) Includes start-up counter of 1024 clock cycles.
(3) 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.
(4) To improve EMI on the LFXT oscillator, observe the following guidelines.
- 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 LFXIN and LFXOUT.
- Avoid running PCB traces underneath or adjacent to the LFXIN and LFXOUT pins.
- Use assembly materials and processes that avoid any parasitic load on the oscillator LFXIN and LFXOUT pins.
- If conformal coating is used, ensure that it does not induce capacitive or resistive leakage between the oscillator pins.
(5) 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.
(6) This represents all the parasitic capacitance present at the LFXIN and LFXOUT terminals, respectively, including parasitic bond and package capacitance. The effective load capacitance, CL,eff can be computed as CIN × COUT / (CIN + COUT), where CIN and COUT are the total capacitance at the LFXIN and LFXOUT terminals, respectively.
(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) Maximum frequency of operation of the entire device cannot be exceeded.
(9) 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}, CL,eff = 6 pF
- For LFXTDRIVE = {2}, 6 pF ≤ CL,eff ≤ 9 pF
- For LFXTDRIVE = {3}, 9 pF ≤ CL,eff ≤ 12.5 pF
Table 5-5 lists the characteristics of the HFXT.