IMX7DCEC datasheet(43/158 Pages) NXP

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Electrical characteristics

i.MX 7Dual Family of Applications Processors Datasheet, Rev. 5, 07/2017

NXP Semiconductors

The system crystal oscillator consists of a Pierce-type structure running off the digital supply. A straight

forward biased-inverter implementation is used.

4.4.2

OSC32K

This block implements an internal amplifier, trimable load capacitors and a resistor that when combined

with a suitable quartz crystal implements a low power oscillator.

In addition, if the clock monitor determines that the OSC32K is not present then the source of the 32 kHz

clock will automatically switch to the internal relaxation oscillator of lesser frequency accuracy.

CAUTION

The internal RTC oscillator does not provide an accurate frequency and is

affected by process, voltage and temperature variations. NXP strongly

recommends using an external crystal as the RTC_XTALI reference. If the

internal oscillator is used instead, careful consideration must be given to the

timing implications on all of the SoC modules dependent on this clock.

The OSC32k runs from VDD_SNVS_1p8_CAP, which is regulated from

VDD_SNVS. The target battery is an ~3 V coin cell for VDD_SNVS and

the regulated output is ~1.75V.

4.5

I/O DC parameters

This section includes the DC parameters of the following I/O types:

Table 28. OSC32K Main Characteristics

Min

Typ

Max

Comments

Fosc

—

32.768 KHz

—

This frequency is nominal and determined by the crystal selected. 32.0 K

would work as well.

Current consumption

—

350 nA

—

The typical value shown is only for the oscillator, driven by an external

crystal. If the interrelaxation oscillator is used instead of an external crystal

then approximately 250 nA should be added to this value.

Bias resistor

—

200 M

This is the integrated bias resistor that sets the amplifier into a high gain

state. Any leakage through the ESD network, external board leakage, or

even a scope probe that is significant relative to this value will debias the

amp. The debiasing will result in low gain and will impact the circuit's ability

to start up and maintain oscillations.

Target Crystal Properties

Cload

—

10 pF

—

Usually, crystals can be purchased tuned for different Cload. This Cload

value is typically 1/2 of the capacitances realized on the PCB on either side

of the quartz. A higher Cload will decrease oscillation margin but increases

current oscillating through the crystal. The Cload is programmable in 2 pF

steps.

ESR

—

50 K

—

Equivalent series resistance of the crystal. Choosing a crystal with a higher

value will decrease oscillating margin.

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