What is an Eye Diagram? Simply put, it is an oscilloscope display of a digital signal that is triggered on both rising and falling edges using a high persistence setting so that the waveforms are all displayed together.
This is all well and good but how does this help us? To answer that question we need to ask ourselves when we are sending a digital signal between components in our system, say a Microprocessor and external memory for example, what are the important signal characteristics to ensure the receiver correctly interprets the data we send on the PCB?
Let's take a simple example, let's assume we are sending the following byte of data from the Microprocessor to an external memory chip: "01101100".
From the memory side we need to ensure we receive the same sequence, and how do we ensure that happens? It all comes down to the all important Signal Integrity!!
What is Signal Integrity?
Signal Integrity refers to measurements which represents the quality of a signal.
The simplest measurement that we would all be familiar is the voltage level, for a digital signal we need to ensure the voltage for which the receiver decodes a '1' and a '0' is reached. This may potentially be say 2V to 3.5V for a '1' and below 2V for a '0' using 3.3V logic.
Using the above example, we need to then ensure that the receiver, the external memory, see's the following voltage levels in time: "0-2V", "2.7-3.5V", "2.7-3.5V" "0-2V", "2.7-3.5V", "2.7-3.5V", "0-2V", "0-2V".
The other important measurement is the received value in time, you see if a '1' doesn't arrive within a clock period the receiver may interpret the bit as a '0'!!! This can happen if there is too much input capacitance at the receiver and the signal is high speed and so the voltage does not rise to at least 2V within the period. For this we can measure the rise time and fall time of a signal. There are many many definitions for these two measurements but in short it is just a measurement for how long a signal takes to rise from 0V to a known percentage of the maximum value, say 90%, and fall time is then the time in the other direction, from the high to the low.
The eye diagram is then a visual representation of both of these all important measurements, at a glance we can see a digital waveform constantly changing from a '1' -> '0' and '0' -> '1' to see if the values have settled to the desired state in time as well as hit the required voltage level. The vertical axis allows us to verify the voltage and the horizontal axis allows us to verify that the rise and fall times are low enough for the receiver to correctly sample the data in time.
This is a very gentle introduction to the all important concept of Signal Integrity which requires an entire course for proper treatment, more on this in the future!