AM6548ACD datasheet(160/284 Pages) TI

160 / 284 page

Note

The load capacitors, Cf1 and Cf2 in Figure 7-25, should be chosen such that the below equation is

satisfied. CL in the equation is the load specified by the crystal manufacturer. All discrete components

used to implement the oscillator circuit should be placed as close as possible to the associated

oscillator WKUP_LFOSC0_XI, WKUP_LFOSC0_XO, and VSS pins.

CL=

C C

(C +C )

SPRS932_CLOCK_03

Figure 7-25. Load Capacitance Equation

The crystal must be in the fundamental mode of operation and parallel resonant. Table 7-22 summarizes the

required electrical constraints

Table 7-22. WKUP_LFOSC0 Crystal Electrical Characteristics

NAME

DESCRIPTION

MIN

TYP

MAX

UNIT

Parallel resonance crystal frequency

32768

Cf1

Cf1 load capacitance for crystal parallel resonance with Cf1 = Cf2

Cf2

Cf2 load capacitance for crystal parallel resonance with Cf1 = Cf2

Cshunt

Shunt capacitance

1.35

ESR

Crystal effective series resistance

kΩ

When selecting a crystal, the system design must consider the temperature and aging characteristics of a based

on the worst case environment and expected life expectancy of the system.

Table 7-23 details the switching characteristics of the oscillator and the requirements of the input clock.

Table 7-23. WKUP_LFOSC0 Switching Characteristics – Crystal Mode

NAME

DESCRIPTION

MIN

TYP

MAX

UNIT

fxtal

Oscillation frequency

32768

tSX

Start-up time

(1)

In order to meet the start-up time defined in this table, the crystal needs to be selected according to the following equation:

Tsu = K×Lm/ (Ro-ESR) + Δt

where Lm is crystal motional inductance, RO is the negative resistance of amplifier, ESR is the crystal Effective series resistance and K

is a constant which represents the initial conditions. Δt is the time amplifier takes to reach its bias point after power down is released.

VDDA_WKUP

WKUP_LFOSC0_XO

Time

VSS

VDDA_WKUP (min.)

VDD_WKUP (min.)

VSS

VDD_WKUP

Figure 7-26. WKUP_LFOSC0 Start-up Time

AM6548, AM6528, AM6546, AM6526

SPRSP52B – DECEMBER 2019 – REVISED JUNE 2021

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