米6体育平台手机版

SLASEB7D June 2017 – December 2020

PRODUCTION DATA

8.13.3.1 Low-Frequency Crystal Oscillator, LFXT

over recommended ranges of supply voltage and operating free-air temperature (unless otherwise noted)⁽¹⁾

PARAMETER		TEST CONDITIONS	V_CC	MIN	TYP	MAX	UNIT
I_VCC.LFXT	Current consumption	f_OSC = 32768 Hz, LFXTBYPASS = 0, LFXTDRIVE = {0}, T_A = 25°C, C_L,eff = 3.7 pF, ESR ≈ 44 kΩ	3.0 V		180		nA
		f_OSC = 32768 Hz, LFXTBYPASS = 0, LFXTDRIVE = {1}, T_A = 25°C, C_L,eff = 6 pF, ESR ≈ 40 kΩ			185
		f_OSC = 32768 Hz, LFXTBYPASS = 0, LFXTDRIVE = {2}, T_A = 25°C, C_L,eff = 9 pF, ESR ≈ 40 kΩ			225
		f_OSC = 32768 Hz, LFXTBYPASS = 0, LFXTDRIVE = {3}, T_A = 25°C, C_L,eff = 12.5 pF, ESR ≈ 40 kΩ			330
f_LFXT	LFXT oscillator crystal frequency	LFXTBYPASS = 0			32768		Hz
DC_LFXT	LFXT oscillator duty cycle	Measured at ACLK, f_LFXT = 32768 Hz		30%		70%
f_LFXT,SW	LFXT oscillator logic-level square-wave input frequency	LFXTBYPASS = 1⁽²⁾ ⁽³⁾		10.5	32.768	50	kHz
DC_{LFXT, SW}	LFXT oscillator logic-level square-wave input duty cycle	LFXTBYPASS = 1		30%		70%
OA_LFXT	Oscillation allowance for LF crystals⁽⁴⁾	LFXTBYPASS = 0, LFXTDRIVE = {1}, f_LFXT = 32768 Hz, C_L,eff = 6 pF			210		kΩ
OA_LFXT	Oscillation allowance for LF crystals⁽⁴⁾	LFXTBYPASS = 0, LFXTDRIVE = {3}, f_LFXT = 32768 Hz, C_L,eff = 12.5 pF			300		kΩ
C_LFXIN	Integrated load capacitance at LFXIN terminal⁽⁵⁾ ⁽⁶⁾				2		pF
C_LFXOUT	Integrated load capacitance at LFXOUT terminal⁽⁵⁾ ⁽⁶⁾				2		pF
t_START,LFXT	Start-up time⁽⁷⁾	f_OSC = 32768 Hz, LFXTBYPASS = 0, LFXTDRIVE = {0}, T_A = 25°C, C_L,eff = 3.7 pF	3.0 V		800		ms
t_START,LFXT	Start-up time⁽⁷⁾	f_OSC = 32768 Hz, LFXTBYPASS = 0, LFXTDRIVE = {3}, T_A = 25°C, C_L,eff = 12.5 pF	3.0 V		1000		ms
f_Fault,LFXT	Oscillator fault frequency⁽⁸⁾ ⁽⁹⁾			0		3500	Hz

(1) 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.

(2) 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 DC_{LFXT, SW}.

(3) Maximum frequency of operation of the entire device cannot be exceeded.

(4) 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}, C_L,eff = 3.7 pF
For LFXTDRIVE = {1}, C_L,eff = 6 pF
For LFXTDRIVE = {2}, 6 pF ≤ C_L,eff ≤ 9 pF
For LFXTDRIVE = {3}, 9 pF ≤ C_L,eff ≤ 12.5 pF

(5) This represents all the parasitic capacitance present at the LFXIN and LFXOUT terminals, respectively, including parasitic bond and package capacitance. The effective load capacitance, C_L,eff can be computed as C_IN × C_OUT / (C_IN + C_OUT), where C_IN and C_OUT is the total capacitance at the LFXIN and LFXOUT terminals, respectively.

(6) 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.

(7) Includes start-up counter of 1024 clock cycles.

(8) Frequencies above the MAX specification do not set the fault flag. Frequencies between the MIN and MAX specifications may set the flag. A static condition or stuck at fault condition will set the flag.

(9) Measured with logic-level input frequency but also applies to operation with crystals.