5.15 Crystal Oscillator, LFXT1, Low-Frequency Mode
over recommended ranges of supply voltage and operating free-air temperature (unless otherwise noted) (4)
PARAMETER |
TEST CONDITIONS |
VCC |
MIN |
TYP |
MAX |
UNIT |
fLFXT1,LF |
LFXT1 oscillator crystal frequency, LF mode 0, 1 |
XTS = 0, LFXT1Sx = 0 or 1 |
1.8 V to 3.6 V |
|
32768 |
|
Hz |
OALF |
Oscillation allowance for LF crystals |
XTS = 0, LFXT1Sx = 0,
fLFXT1,LF = 32768 kHz, CL,eff = 6 pF |
|
|
500 |
|
kΩ |
XTS = 0, LFXT1Sx = 0,
fLFXT1,LF = 32768 kHz, CL,eff = 12 pF |
|
|
200 |
|
CL,eff |
Integrated effective load capacitance, LF mode(1) |
XTS = 0, XCAPx = 0 |
|
|
1 |
|
|
XTS = 0, XCAPx = 1 |
|
|
5.5 |
|
|
XTS = 0, XCAPx = 2 |
|
|
8.5 |
|
pF |
XTS = 0, XCAPx = 3 |
|
|
11 |
|
|
|
Duty cycle, LF mode |
XTS = 0, Measured at P1.5/ACLK,
fLFXT1,LF = 32768Hz |
2.2 V, 3 V |
30 |
50 |
70 |
% |
fFault,LF |
Oscillator fault frequency, LF mode(3) |
XTS = 0, XCAPx = 0, LFXT1Sx = 3(2) |
|
|
|
|
|
|
|
|
2.2 V, 3 V |
10 |
|
10000 |
Hz |
(1) Includes parasitic bond and package capacitance (approximately 2 pF per pin). Since the PCB adds additional capacitance it is recommended to verify the correct load by measuring the ACLK frequency. For a correct setup the effective load capacitance should always match the specification of the used crystal.
(2) Measured with logic level input frequency but also applies to operation with crystals.
(3) Frequencies below the MIN specification set the fault flag, frequencies above the MAX specification do not set the fault flag, and frequencies in between might set the flag.
(4) To improve EMI on the low-power LFXT1 oscillator, particularly in the LF mode (32 kHz), the following guidelines should be observed.
- Keep the trace between the MCU 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, ensure that it does not induce capacitive or resistive leakage between the oscillator pins.
- Do not route the XOUT line to the JTAG header to support the serial programming adapter as shown in other documentation. This signal is no longer required for the serial programming adapter.