How to Use Parallel EEPROM for Storing Data

2019-06-13 | By Maker.io Staff

Parallel memory chips have been around for the better part of 40 years and are still used widely in electronic designs including computers, laptops, and SoCs. In this How-To, we will learn how to use parallel EEPROM memory for storing data.

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How to Use Parallel EEPROM for Storing Data

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Parallel versus Serial EEPROM

When it comes to storing information separately from a microcontroller or SoC, there are many different methods available to makers. For those who feel like going entirely retro, there are magnetic tapes in the form of cassette tapes. For those who want to look at more modern solutions, there are plenty of memory chips on the market with different protocols. However, no matter how information is stored, memory storage will always fall into one of two categories: serial or parallel.

Serial memory involves storing digital information one bit at a time and is streamed to and from a memory device, whereas parallel memory stores an entire word of memory in one go. For example, an 8-bit parallel memory chip will store/retrieve one byte of data in one read cycle, whereas an 8-bit serial memory chip will require 8 cycles to either store or retrieve a single byte.

Parallel memory has some advantages and disadvantages when compared to serial memory, and each memory category is the polar opposite to the other.

How to Use Parallel EEPROM for Storing Data

The AT28C64B

In this tutorial, we will use the AT28C64B, which is an 8KB memory chip that stores information even when the power is turned off. One of the advantages of the AT28C64B is that the IC package uses a common industry pin layout that makes it compatible with many other memory chips, which means that if a parallel memory chip replacement is needed, then the AT28C64B can be used.

How to Use Parallel EEPROM for Storing Data

Parallel EEPROM chips like the AT28C64B have different inputs/outputs that all perform different actions.

Ax Pins – Address pins, used to select a specific memory location
I/Ox Pins – Data pins, used to store/retrieve information from the chip
WE Pin – The Write Enable Pin, used to write a byte to a memory location
OE Pin – The Output Enable Pin, used to read a byte from a memory location
CS Pin – The Chip Select Pin, used to select the memory chip for accessing
NC – These are left unconnected (Not Connected)

Memory Locations

Memory chips store data in specific memory locations that can be written and read. Each memory location has a unique location, and this location has a number that can be selected by using the address pins. The table below shows how the address pins can be changed to select different memory locations:

How to Use Parallel EEPROM for Storing Data

Writing / Reading Data Prerequisites

Before data can be written or read, two things need to be done first - we must select the chip and set the address pins. While the address pins are active high, the chip select is active low; this means when the chip is not being used, the chip select needs to be set to VCC. When the chip select is pulled to ground, the chip becomes selected. Both the WE and OE pins are active low and therefore should always be held at VCC until data is to be accessed.

Reading Data from an EEPROM

Reading data from an EEPROM is very simple and only requires that the address pins are set to the desired address, the chip select is held low, and the OE pin is held low. When these conditions are met, the I/O pins output the byte stored at the address pointed to by the address pins.

How to Use Parallel EEPROM for Storing Data

Writing Data to an EEPROM

Writing data to an EEPROM is also simple and requires the following steps:

Set the address to write to by setting the address pins
Set the I/O pins to the data that is to be stored
Set CS to GND
Set WE to GND
Wait a few seconds
Set WE to VCC

How to Use Parallel EEPROM for Storing Data

Simple Arduino Mega Example

Parallel memory chips often have many address pins, so in our Arduino example, we will connect most of the address pins to ground and only use 2 address pins to give us 4 locations for storing information.

Copy Code

#define CS  2
#define OE  3
#define WE  4
#define IO  PORTC
#define AD0 12
#define AD1 13


void setup() {
  // put your setup code here, to run once:
  
  // Configure control pins
  pinMode(CS, OUTPUT);
  pinMode(WE, OUTPUT);
  pinMode(OE, OUTPUT);

  digitalWrite(OE, HIGH);
  digitalWrite(WE, HIGH);
  digitalWrite(CS, HIGH);
  
  // Configure data pins
  DDRC = 0x00;

  // Configure address pins
  pinMode(AD0, OUTPUT);
  pinMode(AD1, OUTPUT);


}

void loop() {
  
  // **************************************************
  // Write data 0x5A to data location 1
  // **************************************************

  // Configure the control pins 
  digitalWrite(OE, HIGH);
  digitalWrite(WE, HIGH);
  digitalWrite(CS, LOW);
  
  // Set our data on the IO port and configure IO as output
  DDRC = 0xFF;
  IO = 0x5A;

  // Set the address pins
  digitalWrite(AD0, HIGH);
  digitalWrite(AD1, LOW);

  // Perform the cycle
  digitalWrite(WE, LOW);
  delay(1);
  digitalWrite(WE, HIGH);
  digitalWrite(CS, HIGH);



  // **************************************************
  // Read data from data location 1
  // **************************************************

  // Configure the control pins 
  digitalWrite(OE, HIGH);
  digitalWrite(WE, HIGH);
  digitalWrite(CS, LOW);
  
  // Configure data as input
  DDRC = 0x00;

  // Set the address pins
  digitalWrite(AD0, HIGH);
  digitalWrite(AD1, LOW);

  // Perform the read cycle
  digitalWrite(OE, LOW);
  delay(1);
  unsigned char data = PORTC;
  digitalWrite(OE, HIGH);
  digitalWrite(CS, HIGH);
}