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WORKING WITH A LITTLE EE

by George Martin

Start • The Basics • Getting the Routine Down • Debugging Your Reads and Writes • Sources

THE BASICS

The data exchange with the 24C32P is through the SCL (serial clock) and SDA (serial data) pins. The clock signal indicates when data should be transferred to or from the device, and the clock pin serves as a bidirectional output pin from the CPU. The data signal is used to read data from or write it to the device.

When data is written to the memory, the CPU outputs a one or zero to the memory. When data is read from the memory, the CPU writes a high to the data pin and then reads it as an input. The memory drives the data pin either high or low for the CPU to read. Up to eight 24C32P devices can have their clock and data pins tied together and, by using address pins A0, A1, and A2 as hard-wired 1 of 8 decode-logic inputs, each chip's memory can be accessed individually.

Let me point out that, as you read and write this type of memory (flash memory works this way too), there's a lot going on in the background. A write command may actually perform a read-modify-write or an erase-write cycle. You only receive a busy/ready indication, signifying completion of the operation.

THE READ/WRITE SEQUENCE

On powerup, put the device in standby mode by raising the clock and the data lines. In this state, no read or write activity takes place.

The first command written to the device is Start Data Transfer, which is activated by lowering the data line while the clock is high. After you've lowered the clock line, set the data line (either high or low), and then give the last command, Stop Data Transfer, which raises the data line. One bit is then transferred to or from the device. This serial sequencing continues until all the data is transferred.

Let's look at the requirements for reading a byte of data. The read sequence consists of the following operations:

Send Start Data Transfer
Send Control Byte
Send Address Byte 1
Send Address Byte 0
Read Data Byte
Send Stop Data Transfer/

The write sequence looks very similar:

Send Start Data Transfer
Send Control Byte
Send Address Byte 1
Send Address Byte 0
Write Data Byte
Send Stop Data Transfer/

After every byte transfer, the 24C32P acknowledges the transfer, indicating that it went well. If you don't get an acknowledgement, you can assume there's been an error.

Other types of memory have different command sequences, so don't just blindly type in commands. Instead, look carefully at the datasheet. Also, the 24C32P supports block reading and writing. You only need to write the command and address once. After that, you can speed up transferring large blocks of data by using multiple read or write operations.