LPC5411x
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© NXP Semiconductors N.V. 2018. All rights reserved.
Product data sheet
Rev. 2.1 — 9 May 2018
91 of 105
NXP Semiconductors
LPC5411x
32-bit ARM Cortex-M4/M0+ microcontroller
13.5.1 RTC 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.
13.6 Suggested USB interface solutions
The USB device can be connected to the USB as self-powered device (see
) or
bus-powered device (see
On the LPC5411x, 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.