AM68AX datasheet(141/251 Pages) TI

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VDDA_WKUP

WKUP_OSC0_XO

Time

VSS

VDDA_WKUP (min.)

VDD_WKUP (min.)

VSS

VDD_WKUP

J7ES_WKUP_OSC_STARTUP_04

Figure 7-24. WKUP_OSC0 Start-up Time

7.10.4.1.1.1 Load Capacitance

The crystal circuit must be designed such that it applies the appropriate capacitive load to the crystal, as defined

by the crystal manufacturer. The capacitive load, CL, of this circuit is a combination of discrete capacitors

CL1, CL2, and several parasitic contributions. PCB signal traces which connect crystal circuit components to

WKUP_OSC0_XI and WKUP_OSC0_XO have parasitic capacitance to ground, CPCBXI and CPCBXO, where the

PCB designer should be able to extract parasitic capacitance for each signal trace. The WKUP_OSC0 circuits

and device package have combined parasitic capacitance to ground, CPCBXI and CPCBXO, where these parasitic

capacitance values are defined in Table 7-22.

J7ES_WKUP_OSC_CC_05

PCBXI

PCBXO

PCB

Signal Traces

Crystal Circuit

Components

WKUP_OSC0_XO

WKUP_OSC0_XI

Device

Figure 7-25. Load Capacitance

Load capacitors, CL1 and CL2 in Figure 7-23, should be chosen such that the below equation is satisfied. CL in

the equation is the load specified by the crystal manufacturer.

CL = [(CL1 + CPCBXI + CXI) × (CL2 + CPCBXO + CXO)] / [(CL1 + CPCBXI + CXI) + (CL2 + CPCBXO + CXO)]

To determine the value of CL1 and CL2, multiply the capacitive load value CL by 2. Using this result, subtract the

combined values of CPCBXI + CXI to determine the value of CL1 and the combined values of CPCBXO + CXO to

determine the value of CL2. For example, if CL = 10 pF, CPCBXI = 2.9 pF, CXI = 0.5 pF, CPCBXO = 3.7 pF, CXO =

0.5 pF, the value of CL1 = [(2CL) - (CPCBXI + CXI)] = [(2 × 10 pF) - 2.9 pF - 0.5 pF)] = 16.6 pF and CL2 = [(2CL) -

(CPCBXO + CXO)] = [(2 × 10 pF) - 3.7 pF - 0.5 pF)] = 15.8 pF

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AM68A

SPRSP91 – FEBRUARY 2023

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