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LPC2478 Developer’s Kit - User’s Guide Page 10
Copyright 2011 © Embedded Artists AB
3 LPC2478 OEM Board Design
This chapter contains information about the LPC2478 OEM Board design, both 32-bit and 16-bit
databus width. The schematic can be downloaded in pdf from the support page, and is recommended
to have printed out while reading this chapter.
3.1 LPC2478 OEM Board Schematics
3.1.1 CPU
The core part of the design is the NXP LPC2478 microcontroller. It‟s an ARM7TDMI-S CPU core with a
lot of different peripheral units and on-chip memory (512 KByte FLASH and 96 KByte SRAM). There is
also an external memory bus interface. Page 2 of the schematic contains the CPU section of the
design.
The microcontroller crystal frequency is 12.0000 MHz. This frequency has been selected in order to
allow high execution speed (the PLL can generate a 72 MHz core clock from this) as well as
generating standard clock frequencies for the CAN and USB interfaces. The on-chip UART peripheral
includes a fractional baud rate generator that allows standard baud rates to be generated with low
frequency error.
There is a 32.768 kHz crystal clock for the on-chip real-time clock peripheral unit or RTC for short. The
microcontroller can be placed in a very low power mode while the RTC operates and keeps track of
time. Power for the RTC (during these low power modes) comes from the VBAT input pin. Power is
sourced either from the +3.3V power supply or the external VBAT_IN signal (available on the
expansion connector), depending on which one have highest voltage.
3.1.2 Powering
There is no internal power supply on the board. The LPC2478 contains an internal DC/DC converter to
generate the internal 1.8V power needed by the core. The LPC2478 OEM Board module must be
powered by a single, ex3.3V power supply. The supply must be stable and should have at least
four 22uF bulk capacitor(s) close to the power pins on the expansion connectors.
3.1.3 Analog Inputs/Outputs
The LPC2478 contains analog peripherals that need a reference voltage, which is supplied from the
VREF input pin. This reference voltage must be supplied externally can be the VDDA power (which is
a fi3.3V power supply). The VDDA power is available on the expansion connector under the
name V3A. See schematic for details. A filtered version of analog ground is also available under the
name VSSA.
3.1.4 Ethernet Interface
The LPC2478 have an internal Ethernet MAC peripheral with interface to an external PHY (i.e., the
analog interface to an Ethernet interface). Page 2 of the schematic contains the Ethernet PHY. The
DP83848 external Ethernet PHY from National is used and interfaces the LPC2478 via the RMII
interface.
The PHY is driven by an external 50 MHz clock that is also feed to the LPC2478. This is part of the
RMII interface specification.
The PHY is always enables (can be disabled under program control) but have an external „power
down‟ signal that can be used by an external device. There is also an interrupt signal from the PHY
that can be used to signal link events (like connect/disconnect of a cable). Neither the power down nor
the interrupt signal is used by the LPC2478 OEM Board but the signals are available on the expansion
connector. If the signals are not used, just leave them unconnected.