AM68AX datasheet(145/251 Pages) TI

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VDDS_OSC1

OSC1_XO

Time

VSS

VDDS_OSC1 (min.)

VDD_CORE (min.)

VSS

VDD_CORE

J7ES_AUX_OSC_STARTUP_08

Figure 7-29. OSC1 Start-up Time

7.10.4.1.3.1 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 OSC1_XI

and OSC1_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 OSC1 circuits and device package have

combined parasitic capacitance to ground, CPCBXI and CPCBXO, where these parasitic capacitance values are

defined in Table 7-24.

J7ES_AUX_OSC_CC_05

PCBXI

PCBXO

PCB

Signal Traces

Crystal Circuit

Components

OSC1_XO

OSC1_XI

Device

Figure 7-30. Load Capacitance

Load capacitors, CL1 and CL2 in Figure 7-28, 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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