LPC4370 datasheet(132/150 Pages) NXP

132 / 150 page

LPC4370

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Product data sheet

Rev. 2 — 21 October 2013

132 of 150

NXP Semiconductors

LPC4370

32-bit ARM Cortex-M4/M0 microcontroller

13.3 RTC oscillator

In the RTC oscillator circuit, only the crystal (XTAL) and the capacitances CRTCX1 and

CRTCX2 need to be connected externally. Typical capacitance values for CRTCX1 and

CRTCX2 are CRTCX1/2 = 20 (typical)  4 pF.

An external clock can be connected to RTCX1 if RTCX2 is left open. The recommended

amplitude of the clock signal is Vi(RMS) = 100 mV to 200 mV with a coupling capacitance of

5 pF to 10 pF.

13.4 XTAL and RTCX Printed Circuit Board (PCB) layout guidelines

Connect the crystal on the PCB as close as possible to the oscillator input and output pins

of the chip. Take care that the load capacitors Cx1, Cx2, and Cx3 in case of third overtone

crystal usage have a common ground plane. Also connect the external components to the

ground plain. To keep the noise coupled in via the PCB as small as possible, make loops

and parasitics as small as possible. Choose smaller values of Cx1 and Cx2 if parasitics

increase in the PCB layout.

13.5 Standard I/O pin configuration

Figure 42 shows the possible pin modes for standard I/O pins with analog input function:

• Digital output driver enabled/disabled

• Digital input: Pull-up enabled/disabled

• Digital input: Pull-down enabled/disabled

• Digital input: Repeater mode enabled/disabled

• Digital input: Input buffer enabled/disabled

• Analog input

The default configuration for standard I/O pins is input with pull-up enabled. The weak

MOS devices provide a drive capability equivalent to pull-up and pull-down resistors.

Fig 41. RTC 32 kHz oscillator circuit

002aah148

LPC43xx

RTCX1

RTCX2

CRTCX2

CRTCX1

XTAL

LPC4370 Datasheet(PDF) 132 Page - NXP Semiconductors