The Engineer's Workshop

We first began our journey into learning the I2C protocol three weeks ago. In that post, we learned to write to an external EEPROM over the I2C protocol using nothing more than a datasheet and the Arduino’s built-in Wire library. Before learning to read from that EEPROM, which we will do today, we needed to gain the prerequisite knowledge of how data is stored in memory and how pointers work. From there, we learned how the data stored in these variables is passed along through to functions and what an array really is.

It’s been a daunting few weeks, but we’re finally there. Let’s read the data we wrote to our EEPROM armed with nothing more than the datasheet and the I2C protocol.

1. Review the Datasheet

Using the same strategy as before, we look for the command we’re interested in on the datasheet. Since we last wrote to the EEPROM using a page write, it should be pretty easy to guess that to read off our same data, we probably want a page read (also known as a sequential read).

Going to the Sequential Read section of the datasheet, we’re given the following description:

SEQUENTIAL READ: Sequential reads are initiated by either a current address read or a random address read. After the microcontroller receives a data word, it responds with an acknowledge. As long as the EEPROM receives an acknowledge, it will continue to increment the data word address and serially clock out sequential data words. When the memory address limit is reached, the data word address will “roll over” and the sequential read will continue. The sequential read operation is terminated when the microcontroller does not respond with a zero but does generate a following stop condition (see Figure 12 on page 12).

Atmel: https://www.mouser.com/datasheet/2/268/doc0670-1180619.pdf

From here, the code practically writes itself, we just need to follow the directions Atmel has given us.

2. Write the Preamble:

Per the datasheet: “Sequential reads are initiated by either a current address read or a random address read.” Well, we wrote our data to a specific address, so we want to read from that address, so we’ll initiate the sequential read by using the random read. Referring to the Random Read section of the datasheet:

RANDOM READ: A random read requires a “dummy” byte write sequence to load in the data word address. Once the device address word and data word address are clocked in and acknowledged by the EEPROM, the microcontroller must generate another start condition. The microcontroller now initiates a current address read by sending a device address with the read/write select bit high. The EEPROM acknowledges the device address and serially clocks out the data word. The microcontroller does not respond with a zero but does generate a following stop condition (see Figure 11 on page 12).

Atmel: https://www.mouser.com/datasheet/2/268/doc0670-1180619.pdf

The above-boxed section represents the “the device address word and data word address are clocked in and acknowledged by the EEPROM” portion. We’ll start by coding this section. Thankfully, we’ve already done it in the Page Write post- it’s everything in section 2:

Wire.beginTransmission(0b1010000);
Wire.write(0b0000000); // 7 bits of 0s; this method takes a byte though so it will still transmit a byte's worth of 0s.
Wire.write(0b00000000);
Wire.endTransmission();

Boom. Header done.

3. Read the Data:

RANDOM READ: A random read requires a “dummy” byte write sequence to load in the data word address. Once the device address word and data word address are clocked in and acknowledged by the EEPROM, the microcontroller must generate another start condition. The microcontroller now initiates a current address read by sending a device address with the read/write select bit high. The EEPROM acknowledges the device address and serially clocks out the data word. The microcontroller does not respond with a zero but does generate a following stop condition (see Figure 11 on page 12).

Atmel: https://www.mouser.com/datasheet/2/268/doc0670-1180619.pdf

The start condition (with sending a device address) is common to the I2C protocol and is thankfully handled by the Wire library with the simple Wire.requestFrom method, where the first argument is the device address and the second argument is the number of bytes to request from the EEPROM:

Wire.requestFrom(deviceaddress,2);

Since I only wrote two bytes “Hi” to the address, I’m only requesting two bytes back.

Now to actually read the data, we’ll use a simple while loop:

while(Wire.available()) {
    Serial.print((char)Wire.read());
}

Putting together the full code in its entirety from the past two tutorials:

Here is what she looks like on the Serial Monitor:

In the future, we’ll eventually revise this code to make it more versatile by putting it in a library.

In a previous post, we covered how to expand your number of analog inputs by using an external ADC over the SPI bus. Today I want to demonstrate how to use the I2C protocol while simultaneously teaching you how to read a datasheet. After all, there will come a day when you want to use a new device and a library won’t already exist for it.

First of all, let’s establish the Arduino library we will be implementing to communicate with the AT24C256 EEPROM. For this tutorial, we will be using the Wire library to implement the I2C protocol (Mental shortcut: I2C protocol = Wire library). Let’s begin!

1. Review the Datasheet

First, let’s survey the land and have a look at the AT24C256 datasheet. When I look at a datasheet, one of the first things I do is figure out what commands I want to use. In this case, I know I want to do a page write. A page write allows you to write multiple bytes of data all at once, as opposed to a byte write which is all sorts of tedious. I’m a visual learner so once I know what command I want to use, I jump to the figure that parses it out. The figure for Page Write is shown below:

Just looking at this figure I see that every I2C message starts with three words: a device address, a first word address, and a second word address. This preamble is then followed with the actual data you want to write.

2. Writing Out the Code for the Address (i.e. the preamble)

From that diagram alone the code practically writes itself. It’s just a matter of knowing which methods to call in the Wire library. Let’s implement the code one word at a time:

Device Address:

Focusing on this part of the diagram, we see that the device address begins with a 1 (the line is high), followed by a 0 (line is low), 1, and so on so that we ultimately end up with 0b1010000. The last two least significant bits (LSBs) are 0s but can actually be changed to allow for up to four of these EEPROMs to be on the I2C bus. This is also shown separately in the datasheet:

“But wait!” you say, “That’s only 7 bits. A byte is eight bits. What gives?!”

Well, you are technically correct- to make the full byte, we should have an eighth bit that tells the device whether we want to read or write (the R/W in the LSB position shown above), but we are using the Arduino Wire library which expects a 7-bit address. If you are given an 8-bit address like in Figure 7 above, you need to bit shift one position to the right to get a 7-bit address. Notice that you don’t have to do this minor bit of mental gymnastics if you just look at the original diagram in Figure 9. The Wire library will handle appending the R/W bit based on what method you end up calling.

So to begin our I2C transmission, we simply call the following:

Wire.beginTransmission(0b1010000);

First Word Address:

Let’s just assume we’re starting off with a blank slate. So we’re just going to start from an initial address of 0. Honestly, we might as well go ahead and address the second word address here while we’re at it. We can think of the first and second word address as one giant address: i.e. two bytes together (a.k.a. an int). Looking at where the MSB of this “giant” 16-bit would fit in the diagram above (where the * is), we see that this value is a “DON’T CARE BIT”. Since I’m using a 256K EEPROM, our MSB for our address begins where the cross is (so we actually have a 15-bit address). Therefore our 15-bit address comes out to: 0b000000000000000 (i.e. 15 0’s).

Getting back to the first word address, we can see that that the AT24C256 still expects a full 8-bit byte even if it doesn’t particularly care what that first bit actually is. We can accomplish this with the following code:

Wire.write(0b0000000); // 7 bits of 0s; this method takes a byte though so it will still transmit a byte's worth of 0s.

Second Word Address:

Now we just transmit the second word address (i.e. the remaining 8 bits of our 15 bits from above):

Wire.write(0b00000000);

Granted this was a pretty trivial address, but I think you get the idea.

3. Write the Data

Now, I assume you probably didn’t come here to just address the EEPROM. You want to actually write some data to it right? Well, now you can do that using the same Wire.write calls we’ve done before. We’re finally in this part of the Page Write diagram:

This part is rather simple compared to what we’ve done so far. To write data we simply use Wire.write again:

Wire.write(15);

The beauty of using Page Write is that we aren’t limited to just writing one byte at a time (which is what happens when you use Byte Write). With Page Write, once you’ve queued up the preamble, you can then queue up an entire array all at once. For example:

Wire.write("ABC");

4. Putting It All Together:

Putting it all together, the hard-coded Page Write function we just wrote could look something like this:

void writePage(){
  Wire.beginTransmission(0b1010000);
  Wire.write(0b0000000);
  Wire.write(0b00000000);
  byte a = Wire.write("Hi");
  Wire.endTransmission();
}

Don’t forget the Wire.endTransmission(); at the end so that we actually send out our data over the I2C bus. Also don’t forget to designate your Arduino as the master device in your setup block with Wire.begin();.

This tutorial has become a bit longer than I originally intended. Let’s call it a night and we’ll pick back up in the next post with how to request (read) data back off the EEPROM!