Step 2: How to Select Your Serial EEPROM

When choosing the correct EEPROM for your application, a variety of system design decisions must be taken into account. These decisions are based upon the answers to the following questions:

• How many serial bus drivers are available on the microcontroller?

• How many I/O ports are available on the microcontroller?

• What are the different bus interfaces in the other system peripherals?

• How much memory is required for the application?

• What is the maximum speed of the serial communications bus?

• In order to determine the best package for the application, what is the physical size available?

• Finally, are there any specific voltage or temperature constraints?

When choosing a Serial EEPROM, there is a balancing act between cost and density, but the system software application will usually drive this requirement. Usually the amount of memory required to complete a project is estimated before the design is completed. The final density purchased is usually 50% larger to allow for future growth.

In order to understand the process in selecting a specific EEPROM device, an example of the types of questions that come up in the design phase is shown below:

1. Does the microcontroller have a programmable serial port that can support I²C, SPI, Microwire or UNI/O™ serial buses?

2. How many spare I/O ports on the microcontroller are available for use by the EEPROM?

(a) Is your Microcontroller pin limited?

(b) Is it cheaper to switch to a bigger microcontroller or reduce the I/O ports required to interface with the serial EEPROM?

(c) Do you need to use less I/O pins with memory on the microcontroller to free up the extra I/O pins for other applications?

3. Are there any other peripherals in the system design that use a serial interface such as I²C, SPI or Microwire?

(a) If no serial ports are available, will the serial interface be emulated by bit banging several I/O ports?

4. How much memory will be required to complete the project and will it be necessary to upgrade to a larger memory size in the future??

(a) If more memory is needed, will a larger package impact the design?

(b) If more memory is needed will cascading serial EEPROMs be a viable option

5. Is the bus speed known, so that a bus type can be selected?

6. If there is a limitation of I/O ports, will the slower speeds of the I²C, UNI/O^® bus meet the system requirements, since they use the fewest I/O ports, two and one respectively?

7. Are there any space constraints in the application that dictates a smaller package or die/wafer?

8. Is there a minimum or maximum voltage or operating temperature requirement?

When selecting the correct Serial EEPROM, the most important requirements that drive the selection are the types of serial ports or the number of I/O ports available on the microcontroller. The final bus type is a decision that is usually dictated by the microcontroller or the application.

For an SPI EEPROM a 4-pin interface to the microcontroller is necessary. This includes a data input port, a data output port, a clock port and a separate I/O port for the Chip Select of each SPI device. If only 2 I/O ports are available, the SPI or Microwire EEPROMs cannot be selected. Therefore, the bus type that would work best is the I²C or the single I/O UNI/O™ bus as long as it meets the bus speed and density requirements.

In many cases, the application notes and technical briefs you read will point you directly to the part you should use. Finally, contact your local Microchip Sales office and discuss your findings with one of our knowledgeable sales staff or Field Applications Engineers (FAEs). They would be happy to help you with your Serial EEPROM selection.

Step 3 : Cross Referencing Your Serial EEPROM =>