Datasheet

LPC540xx All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2018. All rights reserved.
Product data sheet Rev. 1.8 — 22 June 2018 152 of 168
NXP Semiconductors
LPC540xx
32-bit ARM Cortex-M4 microcontroller
13.6.1 XTAL Printed Circuit Board (PCB) design guidelines
• Connect the crystal and external load capacitors on the PCB as close as possible to
the oscillator input and output pins of the chip.
• The length of traces in the oscillation circuit should be as short as possible and must
not cross other signal lines.
• Ensure that the load capacitors CX1, CX2, and CX3, in case of third overtone crystal
usage, have a common ground plane.
• Loops must be made as small as possible to minimize the noise coupled in through
the PCB and to keep the parasitics as small as possible.
• Lay out the ground (GND) pattern under crystal unit.
• Do not lay out other signal lines under crystal unit for multi-layered PCB.
13.7 Suggested USB interface solutions
The USB device can be connected to the USB as self-powered device (see Figure 49) or
bus-powered device (see Figure 50
).
On the LPC540xx, the USB_VBUS pin is 5 V tolerant only when V
DD
is applied and at
operating voltage level. Therefore, if the USB_VBUS function is connected to the USB
connector and the device is self-powered, the USB_VBUS pin must be protected for
situations when V
DD
= 0 V.
If V
DD
is always at operating level while VBUS = 5 V, the USB_VBUS pin can be
connected directly to the VBUS pin on the USB connector.
For systems where V
DD
can be 0 V and VBUS is directly applied to the VBUS pin,
precautions must be taken to reduce the voltage to below 3.6 V, which is the maximum
allowable voltage on the USB_VBUS pin in this case.
One method is to use a voltage divider to connect the USB_VBUS pin to the VBUS on the
USB connector. The voltage divider ratio should be such that the USB_VBUS pin is
greater than 0.7 V
DD
to indicate a logic HIGH while below the 3.6 V allowable maximum
voltage.
For the following operating conditions
VBUS
max
= 5.25 V
V
DD
= 3.6 V,
the voltage divider should provide a reduction of 3.6 V/5.25 V or ~0.686 V.