User`s guide

LPC3250 Developer’s Kit - User’s Guide
Page 10
Copyright 2011 © Embedded Artists AB
3.1.1.6 I2C E2PROM
There is a 256 kbit E
2
PROM accessible via the I
2
C interface (I2C1). The LPC3250 has two on-chip I
2
C
communication channels (I2C1 and I2C2). More peripheral units are easily connected to the two-wire
I
2
C bus, just as long as the addresses do not collide. The address of the 256kbit E
2
PROM is 0xA0,
which is also indicated in the schematic.
There are 2.2 kohm pull-up resistors (pull-ups are always needed on I
2
C busses) on the board on both
I
2
C channels.
3.1.2 Schematic Page 3: Powering
3.1.2.1 1.2V and 1.8V Fixed Voltages
The LPC3250 requires three fixed voltages; 1.2V for the core, 1.8V for the memory interface and 3.15-
3.3V for the rest of the i/o interfaces. The 1.2V and 1.8V voltages are generated by two LM3671MF
step-down switching regulators from National Semiconductor. These regulators are capable of
generating 600mA, which by far exceed the needed current by the LPC3250 and other components on
the LPC3250 OEM Board. The 3.15-3.3V voltage is the input voltage to the LPC3250 OEM Board, see
below.
The Real-time clock also needs a 1.2V power, which is generated by a LDO (MIC5232).
3.1.2.2 1.2V Adjustable Core Voltage
The core 1.2V voltage is adjustable and is generated by the step down switching regulator LTC3447
from Linear Technologies. This regulator is capable of generating 600mA, which also by far exceed the
needed current by the LPC3250. The adjustment is done via an I2C channel, I2C1 in the LPC3250
case. The core voltage can be adjusted as a power save feature. By lowering the voltage (down to
0.9V), the total power consumption can be lowered but the clock frequency of the core must then also
be lowered. This is a trade-off that is important for hand held/portable equipment. Note that it is the
user’s responsibility not to program the LTC3447 to generate too high core voltage, which is
possible. The LTC3447 can generate voltages up to 2V, which by far exceed the limits for the
LPC3250.
See the LPC3250 datasheet for exact details about voltage ranges, but it is in the region of 1.1-1.39V.
Also see the LTC3447 datasheet for details about how to adjust the voltage (it is a write-only register).
The I2C address for the LTC3447 is indicated in the schematic.
Note that a core voltage of 1.35V should be programmed when working with the external DDR
SDRAM.
3.1.2.3 Input Voltage
The input voltage to the LPC3250 OEM Board is given by the requirements of the LPC3250. The
recommended input voltage range is 3.15V to 3.3V. The input supply must be stable but there are no
special needs for bulk capacitors close to the power pins on the expansion connectors. The needed
capacitors are placed close to the switched step down switching regulators on the LPC3250 OEM
Board.
Note that the LPC3250 OEM Board is sensitive to input noise on the input voltage. The peak-to-peak
noise should be below 10mV. A linear regulator to feed the input voltage is strongly recommended.
3.1.3 Schematic Page 4: External Memories
Page 4 of the schematic contains the external memory interface and the external memories. The
memory interface uses a 16-bit databus and operates at 1.8V level, which minimizes power
consumption.