LPC18S50 datasheet(133/151 Pages) NXP

133 / 151 page

LPC18S50_S30_S10

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Product data sheet

Rev. 1 — 23 February 2015

133 of 151

NXP Semiconductors

LPC18S50/S30/S10

32-bit ARM Cortex-M3 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. Vi(RMS) must be lower than 450 mV. See Figure 41 for a similar slave-mode

set-up that uses the crystal oscillator.

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.

Ensure that no high-speed or high-drive signals are near the RTCX1/2 signals.

13.5 Standard I/O pin configuration

Figure 44 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 buffer disabled and pull-up

enabled. The weak MOS devices provide a drive capability equivalent to pull-up and

pull-down resistors.

Fig 43. RTC 32 kHz oscillator circuit

002aah066

LPC18xx

RTCX1

RTCX2

CRTCX2

CRTCX1

XTAL

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