uC Ethernet - Getting Started
This is my first project for the web site. From a hardware point of view it is probably the simplest.
As my current interest is in LPC21xx micro's I thought it would be nice to have a project that would involve
firmware and a bit hardware design. I have, anLPC2106 eval board purchased from
sparkfun, I though it would be a good idea to provide some extra hardware for it,
to enable the LPC2106 to be Ethernet enabled. I bought LPC2106 eval board to learn about programming ARM7 based micro's. The LPC2106 seemed like a good starting point
and the eval board was within my budget.
Adding hardware to the LPC2106 eval board to enable it to be ethernet
ready is a relatively simple task. All that is required
is a Media Access Controller (MAC), isolation transformer and RJ45 jack. To actually get the eval board to talk over the ether
requires effort if you start from scratch, fortunately a good chunk of this
work had been done by various embedded boffins.
There are a huge number MAC IC's. The majority are geared towards the PC market and
use a PCI or IDE bus interface, for a small micro this would not be practical
as all or most of the I/O pins would be used for communications. Microchip,
however, have produced an Ethernet MAC with a low I/O pin count micro in mind, which
is great news.
The ENC28J60 was chosen for this project. It supports a 20 MHz SPI interface for
communicating with a microcontroller, thus in theory only 4 pins could be used to
communicate with the IC. A fifth line is available and can be used to generate interrupts, this makes things easier? / quicker when programming the micro. The ENC28J60 comes in various sizes, to make the routing
relatively easy but keep the board at a reasonable size I have chosen to go for
the SO package.
Now that I have a suitable MAC IC, the IO requirements need to be understood so
I can correctly select all external components.
First, power requirements. This is easy, minimum of 3.1V required at a maximum of
180mA. The LPC2106 eval board is capable of supplying 3.3 V @ < 800mA. Some of this
current will be consumed by the eval board but it should be less 50 mA.
I/O signal levels between the LPC2106 an ENC28J60 are compatible, in fact the ENC28J60
has 5V tolerant inputs. The Ethernet I/O lines should me coupled to the Ethernet
using a 1:1 turns ration transformer as detailed in the datasheet. To keep the size
of the PCB small and keep cost low RJ45 jacks can be purchased with transformers built in.
The other various input and output pins are used to setup the IC so it operates
in the correct manner, it is all straight forward if you follow the data sheet. So now all the major components have been identified the schematic can be drawn...
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