46
SLAS826F – MARCH 2015 – REVISED MARCH 2017
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Specifications
Copyright © 2015–2017, Texas Instruments Incorporated
5.25.3 Clock Specifications
lists the input requirement for the low-frequency crystal oscillator, LFXT.
(1)
Does not include board parasitics. Package and board add additional capacitance to C
LFXT.
Table 5-7. Low-Frequency Crystal Oscillator, LFXT, Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
ESR
Crystal equivalent series resistance
f
OSC
= 32.768 kHz
16
40
65
k
Ω
C
LFXT
Capacitance from LFXT input to ground and from
LFXT output to ground
(1)
7.4
12
24
pF
C
SHUNT
Crystal shunt capacitance
0.6
0.8
1.6
pF
C
m
Crystal motional capacitance
1
2
10
fF
lists the characteristics of the low-frequency crystal oscillator, LFXT.
(1)
Total current measured on both AVCC and DVCC supplies.
(2)
Measured at ACLK pin.
(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 DC
LFXT, 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}, C
L,eff
= 3.7 pF.
•
For LFXTDRIVE = {1}, 6 pF
≤
C
L,eff
≤
9 pF.
•
For LFXTDRIVE = {2}, 6 pF
≤
C
L,eff
≤
10 pF.
•
For LFXTDRIVE = {3}, 6 pF
≤
C
L,eff
≤
12 pF.
Table 5-8. Low-Frequency Crystal Oscillator, LFXT
over recommended ranges of supply voltage and operating free-air temperature (unless otherwise noted)
PARAMETER
TEST CONDITIONS
V
CC
MIN
TYP
MAX
UNIT
I
VCC,LFXT
Current consumption
(1)
f
OSC
= 32.768 kHz,
LFXTBYPASS = 0, LFXTDRIVE = {0},
C
L,eff
= 3.7 pF, Typical ESR and C
SHUNT
3.0 V
100
nA
f
OSC
= 32.768 kHz,
LFXTBYPASS = 0, LFXTDRIVE = {1},
C
L,eff
= 6 pF, Typical ESR and C
SHUNT
120
f
OSC
= 32.768 kHz,
LFXTBYPASS = 0, LFXTDRIVE = {2},
C
L,eff
= 9 pF, Typical ESR and C
SHUNT
150
f
OSC
= 32.768 kHz,
LFXTBYPASS = 0, LFXTDRIVE = {3},
C
L,eff
= 12 pF, Typical ESR and C
SHUNT
170
f
LFXT
LFXT oscillator crystal
frequency
LFXTBYPASS = 0
(2)
32.768
kHz
DC
LFXT
LFXT oscillator duty cycle
f
LFXT
= 32.768 kHz
(2)
30%
70%
f
LFXT,SW
LFXT oscillator logic-level
square-wave input frequency
LFXTBYPASS = 1
(3) (4)
10
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
(5)
LFXTBYPASS = 0, LFXTDRIVE = {1},
f
LFXT
= 32.768 kHz, C
L,eff
= 6 pF
200
240
k
Ω
LFXTBYPASS = 0, LFXTDRIVE = {3},
f
LFXT
= 32.768 kHz, C
L,eff
= 12 pF
300
340