RT600 datasheet(137/167 Pages) NXP

137 / 167 page

RT600

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Product data sheet

Rev. 2.0 — 1 April 2022

137 of 165

NXP Semiconductors

RT600

Dual-core microcontroller with 32-bit Cortex-M33 and Xtensa HiFi4

Audio DSP CPUs

16.4 RTC oscillator

In the RTC oscillator circuit, only the crystal (XTAL) and the capacitances Cx and Cy need

to be connected externally on RTCXIN and RTCXOUT. See Figure 33 “RTC oscillator

components”.

For best results, it is very critical to select a matching crystal for the on-chip oscillator.

Load capacitance (CL), series resistance (RS), and drive level (DL) are important

parameters to consider while choosing the crystal. After selecting the proper crystal, the

approximate external load capacitor Cx and Cy values can also be generally determined

by the following expression:

Cx = Cy = 2CL - CPad - 2CSTRAY

Where:

CL - Crystal load capacitance

CPin - Pin capacitance of the RTCXIN and RTCXOUT pins (~3 pf per pin).

CSTRAY – stray capacitance between RTCXIN and RTCXOUT pins.

For example:

CL = 9 pF

Cx = Cy = 2CL - CPad - 2CSTRAY

Cx = Cy = 2*9 - 3 - 0 = 15 pF.

Although CSTRAY can be ignored in general, the actual board layout and placement of

external components influences the optimal values of external load capacitors. Therefore,

it is recommended to fine tune the values of external load capacitors on actual hardware

board to get the accurate clock frequency. For fine tuning, output the RTC Clock to the

CLKOUT pin and optimize the values of external load capacitors for minimum frequency

deviation.

The RTC oscillator can be bypassed and driven by an external signal. To accomplish this,

set bit 8 to 1 in RTC control register (RTC_OSC_PD), the RTCXOUT pin is left

disconnected (floating), and the RTCXIN pin is driven by an external source with a level

Fig 33. RTC oscillator components

aaa-025723

RTxxx

RTCXIN

RTCXOUT

CX2

CX1

XTAL

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